depends whether they are set too tight or loose for a prolonged period. Always best to err a bit on the loose versus tight side....mostly for heat transfer...will also marginally decrease overlap and modestly diminish absolute lift. Visualize a loose valve as the lobe goes whirling into the eccentric and the valve gets lifted a bit later and higher up on the lobe in essence doing a bit less work...and getting off the lobe a tad sooner.
I generally run my valves on my M-30 about a slip .013" both intake and exhaust set cold. It is rare to burn a valve on an M-30 even set too tight...but generally the same amount tight as loose is worse case scenario. It is also a fallacy that valves just become loose over time...they can tighten a bit as well.
HTH,
George