Is he giving line notes during the rehearsal itself (ie, stopping a scene to correct speech) or making notes and giving them after the scene is finished? If the latter, it's probably never a bad idea to correct bad habits from forming. If the former, though, I'd have issues: if the actor is not supposed to be offbook yet, and is still working things out in his or her head, having constant interruptions to correct minor line flubs could be detrimental to getting through a scene and focusing on the acting.
There's also the question of line corrections once you go into tech/performances. If the actors get used to having notes from the director, it may be difficult for them to adjust to getting notes from you once he is no longer there to do so (assuming the director will not be there for the entire run). I prefer to establish that sort of "chain of command" early on to prevent confusion as we go through the process.