Depends on the rake and trail as well as speed. My current MTB (Moots Rigormootis, fwiw) is very quick steering, can't be left alone for a second. My previous one (GT Xizang) was built for an old short travel fork but had a more recent longer fork which I think may have slackened the head angle. Whatever the reason, it was more stable.

Here's the thing - although all of my bikes will do trackstands - *not one* of them will do wheelies. This may be completely caused by the rider.