depends on the type of steel, thickness, and application. high alloy and/or medium carbon steel, or something 1/2" or thicker i'd think would need a preheat

ASM Handbook has a very detailed section on hardfacing. Your answer may depend entirely on what you are hardfacing and the process you intend to use. My personel experience is limited to large industrial equipment and the SMAW, GMAW processes. We hardface extremely large loader buckets and have never preheated and have never had any problems. I know that hardfacing on these buckets will often crack, but this is not considered a problem on loader buckets.

Mike Sherman
Shermans Welding

I would consult with whoever makes your hard facing materials. Most manufacturers have data that should answer your questions.
CHGuilford