Safe to use 10-speed wheel in a aluminum frame bike made for a 7-speed
With aluminum, dropouts cracking from the fatigue caused by this is a real thing that can happen. It may take a long time, or it may not in the case of a frame with a long life of fatigue already on it. There are a lot of factors like rider strength/weight and how good the dropout alignment was to begin with. 

Chunky dropouts can somewhat mitigate the risk, but the problem there is the axle will flex instead of the dropouts, which can ruin bearing life and cause drag.