In my experience, if the WPS calls for preheat I preheat. I would suggest a local preheat to the area to be tacked.
The preheat will reduce the likelyhood of the tacks cracking.
The requirements you "MUST" follow are probably covered in the welding procedure and the applicable code. If there is not a welding procedure, I would suggest developing one.
The thickness of the base metal, welding process and joint geometry all affect the cooling rate of the weld. Thicker metals cool off quicker, GTAW cools of slower than SMAW, a 45 Degree included angle would cool faster than a 70 degree included angle.
Have a nice day
Gerald Austin
Only in the case of thin materials should you take the risk of tacking without pre-heating, but as already stated: - If the WPS calls for it, then you had better do it.
Regards
Niekie