The answer really depends on why you want to do it? Collapsible stocks are Uber-Evil and all that so that makes them tacti-cool thing to have around here but lets not forget that, with a few exceptions, they are generally not all that comfortable to shoot. Now if you need one for a legitimate reason, (i.e. you wear BA, chest rig, thick heavy clothing or have a really short LOP or you need to move around with it in very tight spaces) then obviously go for it. So basically...
If you want to do it because
A. You think they look cool and plan to spend more time looking at it them shooting it.
B. You have a functional reason why you need or could benefit from one.
Then go for it but if neither A nor B applies to you and you just want to enjoy shooting you 20" AR, then get a fixed A1 length stock and you will be happy.
That's my $.02 on it.