Well, I have searched in vain for the reference requiring Instructors to sign every logbook entry, so, unless anyone can come up with anything, I'm going to assume that it no longer exists.
However, DaveW's comments about the difference between 'should' and 'must' notwithstanding, I would advise pilots to ask the Instructor to sign the logbook entries for any instruction they receive between Revalidations, if only because most Examiners will require it for Revalidation.
MJ
