We have our BJ remind the receivers : "Give me a good fair catch signal ". IMHO, if there is contact after the catch, you need to decide : (1) was the signal valid, if not = 5 yards; (2) was there contact after a valid signal and completed catch = 15 yards.
Things to remember:
An invalid signal falls under PSK and would be enforced from where the kick ended, with all-but-one potential if the kick ended in advance of the invalid signal.
Don't confuse contact by K after a successful fair catch with KCI, as this would be a DB personal foul, HOWEVER, R would still have the choice to free kick and where to spot.
I'm with Bossman that the invalid signal call is rare, but recall once when an R player started to wave both arms and yell "FAIR CATCH, FAIR CATCH" on his 10 as the ball bounced behind him on it's way to the end zone. K's coverage let up, and the proper call placed the ball @ R's 5. Be alert for screwy things such as that.