Hmmm sandro... I think checking valves is a maintenance task that needs doing. The book says every 20,000 km.

I have to admit that I've never noticed any improvement in a bike's operation after having done it. If the engine rattles a lot the work may need doing, but the bigger problem imo is if there is not enough clearance. Eventually this can be expensive ie burnt valves.

A few hours of work for peace of mind is worth it, I think.