note that geosurvey teams work..... unrealistically.
Basically, bodies don't have a set amount of minerals to be found - rather, every production cycle (5 days by default) the geosurvey team has a chance to either 1. find nothing and keep working, 2. generate new minerals 3. finish the survey (they can find new minerals and finish the survey on the same turn).
higher skill = higher chance to generate minerals and lower chance to finish the survey, so it means they will magically generate more minerals for you. Since you can't send a higher skilled team in later to find even more (once it's done, it's done forever) you want to use the highest trained team you can.
On the other hand, planets survey faster then moons, and moons survey faster then asteroids (asteroids take forever). So you can either try and send them to asteroids/moons first to train them up slowly (remember, people retire or die after a long enough time) or just send them to poor quality planets first to train up faster, at the expensive of finding less overall minerals.
Personally, I don't bother training them. The total minerals found can end up huge sometimes, but normally even with a 200 - 250 skill team you're not going to find more then 100,000 - 300,000 of a mineral.... when you've got planets with millions to tens of millions of minerals that's just not enough to be worth the hassle.