Basement walls usually bow for the same reason that they crack—water pressure. As a rule of thumb, if the wall is bowed in less than an inch and is no longer moving, it won’t be a problem. You could straighten it out, but this is costly and time consuming, and really not necessary. Just hide it behind the framed walls of your remodel. Of course, if the bow is still moving, the first thing to do is find out why and fix the cause. Professional advice is worth seeking out here.
If a wall is bowed in more than an inch, you probably need to straighten it. I say probably, because I’ve been in basements with the walls bowed in more than a foot. (I didn’t stay long, but I’ve been in them.) They’ve been that way for dozens of years, inching their way in, and at some point, they’ll cave. When you’re investing time and money in a basement remodel, I don’t think it makes sense to gamble that a questionable wall will hold up for years to come.