V50 junior/mid-level associate chiming in with my 2 cents with respect to turning down work.
In my experience, people who turn down work early on are doomed...however, no one at my firm is billing 2400 hours, or even 2200 (non-NYC, major market). The good associates in my group got to where they are by taking on everything they can and killing themselves for the mid-level/senior associates that gave them work as a junior and their reputation for that "team player"/hard-worker mentality spread. If you are really busy and can still successfully squeeze in a rush assignment for someone important (and starting out that means mid-level and senior level associates), they will appreciate it. You bailed them out of a jam, and they begin to trust you. Once you have had a couple of years of good hours, you can begin to push back a little, or hopefully people start to protect you.
My experience could be very different, because my firm doesn't grind associates to a pulp as quickly as many other firms, but I recommend that incoming associates to never turn down work unless they think it is physically impossible to complete the task. Needing to pull an all-nighter or going on 3-4 hours of sleep for a few days does not mean it's physically impossible. The people who say no, go home at 7pm when others are working late, simply do not last. They might hang around for a little while, but they get shit assignments and their hours just scrape by. If you want to do something unrelated to your practice after biglaw then this might suit you fine. If you want to go in-house or stay in a firm setting, your next employer will care about what you did. Hanging out for 4 years at a big firm won't land you a good in-house gig or a good position at another firm. You need to get substantive experience beyond due diligence and drafting basic ancillary documents, or doc review/basic motions.
Of course, every firm is different - that's just the culture at my firm, in my department, in my office.
I will also say that all of this advice depends on how your department is staffed. If you are leanly staffed, you might have more leverage to gently say no if you are truly swamped (and you should judge this based upon how busy others are), because there is so much work to go around you are unlikely to get frozen out. Though, I know at my firm, if you say no to some partners (basically the few most powerful), they will never use you again. Their close-knit team has a little more wiggle room, but not much.
If your department is overstaffed, you are insane to turn down any work at all unless it is physically impossible to complete it (and I mean you have 2 rush assignments that
need to be done in an hour and each will take an hour to complete). Even then, I wouldn't say no... you say "I am pushing hard for a 1pm deadline for xyz, is it okay if I turn to this afterwards?" You also better be damned sure the person you tell that to is less important (generally) or more understanding that the person who assigned you xyz... if not, you do the reverse (call the person who assigned xyz and see if there is flexibility, while explaining to the person who came second that you will see if you have the flexibility and let them know). Generally speaking, many firms are still struggling. I still hear stories about people being laid off after a year. Others lost all of their mid-level associates and are struggling to find warm bodies to fill the ranks (slight exaggeration, but not by much).
I will also say that I whole-heartedly agree that taking on too much and turning in bad work product is much worse than turning down work (which I realize is somewhat in conflict with my above statements). Part of it is learning that deals die or are delayed, cases settle or are delayed, etc., so often 1/4 to 1/3 of the stuff of my plate ends up having more flexibility in the deadline than I was originally told. So, you often have more bandwidth than you might think, especially if you are just starting out and haven't seen this happen yet. Additionally, you will usually have assignments with flexible deadlines that can be rearranged - you just need to be careful to be smart about who you ask for more time and you need to explain why you are asking for more time (wanting to go home at a reasonable time as a 1st or 2nd year is not a good reason to ask for more time). You only really learn this through first-hand experience, or by learning from your elder associates.
Don't be afraid to go to someone a year above you that you trust and is well respected, present them with your predicament (assuming it is a real predicament, and not you whining) and ask for their advice. Chances are that they already encountered this same problem and can offer some tidbits of advice. I am surprised how some incoming associates are afraid to reach out to the 2nd/3rd year associates for advice. This shit is not intuitive, so don't be afraid to ask when it comes to work-style of associates/partners from successful associates. You will know they are successful by their hours, by who they work with, and, obviously, if you hear everyone rave about them. Just be careful to take all advice with a grain of salt, of course. You will get advice from some people who have more physical stamina than you (so you will simply not be able to do what they do), and you will get advice from people who have a more years of experience and can get away with handling things differently or may not care anymore (especially if they are
Looking for Greener Pastures). At the end of the day, a lot of this is luck and situational so you just need to be able to adapt to the cards you are dealt and get what you can out of the opportunity. No one goes through the ranks without getting banged up and bruised, no matter how kind the firm is.
Sorry for the rambling paragraphs... trying to blast this out before work.
