If you could only put a semi or a slick on one tire (Fr. or Bk) which would be most advisable? Or is it a dumb idea and something that should be done in pairs? Im picking up some Ergon E1 grips today, and have a *little*incidental case with which im going to get some grip tape, and maybe a tire-but not two since I can't drop 60.00 on a pair. Can you just do one at at time?