That's reassuring, since there isn't any "variable gravity" until you're worried about altitudes that reach tens or hundreds of km high.
I wonder what he meant?
Well, I've never done it but I'd guess when designing a model it's generally a good idea to cover the variables so that the output is as accurate as possible.
Gravity does vary by altitude, latitude, underlying rock density and so on. 30,000 ft is about 0.3% lower. A rough wiki driven guess suggests total variation may as much as 1%. Whether this is significant or not I don't know.