VERSION:2.1
N:Andrew Kester;PE
FN:Andrew Kester, PE
EMAIL;PREF;INTERNET:akester@cfl.rr.com
REV:20090326T124026Z
END:VCARD
Like others have said, they have minimium joist seats, but you can specify anything you want above that (maybe less also). There are all kinds of situations that dictate different heights, you may choose an odd number to match CMU coursing for example. Pay close attention to the minimium bearing length though, there is less flexibility in that. The best way to do it is in your detail show a working point and then give them a dimension, they will detail it accordingly.
HTH,
Andrew Kester, P.E.
Orlando, Florida
Orlando, Florida