You can fix crooked studs; the time to do it is before you drywall.
Bowed or crooked studs can really wreak havoc with the appearance of your finished drywall job. The defect usually looks even worse after you paint. If you try to get away with covering a crooked stud, even if it doesn't show it can cause trouble down the road: the stud is crooked for a reason and may continue to shift. It also makes hanging pictures and installing shelving units more difficult. Units have even fallen because they missed or were barely attached to a crooked stud. Loose nails or cracks can also appear in the finish and will be a real problem to solve at a later time.
To fix a crooked stud, first find the cause. Is it nailed in the wrong place at the top or bottom?
Using a reciprocal saw with a thin metal cutting blade, cut the nails on the offending end. Just slide the blade in the crack and do not cut the stud. After cutting it loose, move it to the proper place. If you are good at toe nailing, you can just nail the stud back into place. If you need a little help, just cut a two by four the proper length of the spacing. Nail it to the bottom plate and then attach the stud with nails. You can also use this method to force a stud into place.
A word of caution about forcing studs over instead of cutting the nails: do not attempt to force the top or bottom more than about one quarter of an inch. Remember the law of physics about action and reaction; you may be forcing the adjoining stud out of place. While correcting one you could move the other. You can avoid this by nailing a short two by four securely to the bottom plate then driving the offending end toward it.
You can use a similar method for correcting bowed studs but you might have to put pieces from one end of the wall to the other. The bow can pass to the next stud and so on. Usually it is best to completely remove a bowed or twisted stud and replace it with a straight one. The time required to replace it is usually less than becoming involved in other remedies.
Be sure to check all areas when finished for protruding nails and other crooked studs. Like many tasks, preparation is as important as application. By correcting crooked studs before you drywall, you can save a lot of time and work down the road.