I would say that it is a matter of connotation, rather than difference in meaning. "Tepid" has, for me, quite negative connotations (possibly because of other -id words - vapid, insipiod, stupid, etc) whereas "lukewarm has, at the least, a "safe" connotation - you can't scald yourself in it.

And may I extend a far from "likewarm" (Whoops - that should be lukewarm, of course!)and definitely not "tepid" welcome to you, wordplayer.