If one can keep ones head cool it is better to wait for at least 3-4 months. Objectively this goes almost every time. The only RPGs that I have that were reasonably bug-free upon release are the BGs, the IWDs, and Oblivion (which lacked in other areas, but it was at least bug free). Especially open ended RPGs require delicate balancing and checking for script bugs, and unless one builds on a familiar engine (like the infinity engine games post BG1 did) this is difficult to pull of in one go. If you wait for a few months you'll likely see a few patches and maybe a price drop, and maybe even an expansion
That said I dont have the discipline to keep my cool today and wait when encountering a new title from devs I trust, or sequels to titles I like. I got Gothic 3 very soon after release (even though I knew the predecessors took quite some patching to run well). I played through half of the game before patch 1.08 finally balanced the boars...
The list of RPGs that were well patched by the time I got them is luckily longer, though many purchases were staved off by me not meeting hardware requirements (I didnt upgrade my PC any time between 1997 and 2004).
Found in the budget bin and well patched:
BG1 + TotSc (might well be my best value for money ever), BG2 + ToB, Icewind Dale I&II, Arcanum (only took this up seriously last week as I couldnt get past the beginning before), Gothic I&II(was hard to get, I only bought it after finishing BG), Morrowind (due to hardware requirements
Unfortunately even the patched MW crashes A LOT, but it still kicks its sequel in the nuts), Might and Magic VI-VIII, Darklands...
Demise and Gothic 3 are the ones that I got immediately, both were buggy but playable on the initial release.
If we go outside the RPG world to the strategy games that I enjoy they tend to be buggy as hell upon initial release... Paradox Interactive (Europa Universalis, Hearts of Iron, Victoria, Crusader Kings) might be worse than ANY RPG developers when it comes to bug ratio upon release, but they do have a good patching policy. Always wait for 3-4 months before getting one of their games
Waiting also has the benefit that you are more likely to have hardware able to run the game on full settings. The problem for us is however that not that many good RPGs are released, so it is tempting to jump at interesting titles immediately when they come out.
EDIT: I really miss the days when games were bug-free upon release
The gold box games (except Dark Queen of Krynn), Eye of the beholder, etc were always playable out of the box.