Good stuff from Stephan, as usual!  If you were working in my shop / site, I would make a brash statement that yes, preheat is mandatory and maybe even especially where Stellite hardfacing is applied.  However, most things in our business have too many variables to make such brash statements.  You mention seal welds but don't tell us the applications or thickness of materials you're working with... are these seal welds for threaded connections?  Codes often dictate mandatory preheat, interpass and PWHT.  It's a bit too difficult to make a sound call in your instance without sounding brash, as I mentioned earlier.