While the head is off, it would be mental not to at least check the valves.
Marks on pistons will be ok as they are a lot stronger than the valves.
I would be concerend if all four are marked though, has the head been skimmed a silly amount ?
My car was running perfect, but had 16 knackered guides, and eight leaky exhaust valves, as well as knackered dizzy.
tabetha