If they are the original '88 guides, be advised that the material used has been upgraded 3-4 times since your car was built. Also, M103s are tough on guides given the way the valves are actuated - I understand there are a few aggressive angles in there that can be tough on the guides.
If I were you (and I was in the same situation last yr. with my '88 300TE) I would do the guides now, and forget about them for the next 150-200k miles.
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car