Its easy to test for trade and refinery. Put 1 on one planet, put 2 on the next, then 3 on the next. Then compare the income from the drop down list.

There is a practical limit for refineries though, since resource asteriods only support so much activity. But that does vary according to how many resources are around.

Culture centers are different though.

I try to put 1 on all planets. But a planet next to enemy space may be vulnerable to enemy culture. On those, assuming I'm curently playing defensively in that area of the map, I'll put 3 culture. Then I'll send in a scout periodically to check what culture the next planet(s) along are putting out, especially if their colour line is getting close to my planet. If they are putting out a higher culture rate, I try to build more culture on that planet. I put 7 on one planet recently, as the enemy planet next to it had 6.

So culture is more protecting your existing space from culture attacks, and about using it to attack the enemy. Either way, you plan this deliberately.