If you were to buy shimano branded wheels hubs made for 10 speed, then these are 10 speed only (certainly ultegra/DA - not sure if it applies to 105 and lower). The alloy freehubs have a deep spline to protect them cos theyre alloy. A 9 speed cassette wont fit cos the splines on the cassette arent cut deep enough, but 10 cassettes are.
some wheels (eg mavics) can be used for 8/9/10, but there will be a gap between the top of the (low cut) hub spline and the (deep cut) 10 spd cassette. This is ok and designed to work cos its all hard steel. Similar if you put a 10 speed cassette on a shimano 9 speed hub.
to reassure you, Ive had both 9 and 10 speed cassettes on a XT 8/9 hub and on some ksyrium wheels.
hope it makes sense, Im a bit of a clumsy writer.
A bicycle can't stand up on its own as it's two tyred