Don't get involved. Let the owning team of the ball do it.
I disagree here SM. Before the game, teams will supply me with at least two match balls (pref. a bag of all the ones being used during the game, no more than 4 or 5). I or one of my assistants will check the ball pressures with a gauge, get it to want we want, keep one, and then put the rest by the dugouts (or give them back to the manager if lower leagues).
If you want something done well, do it yourself.
When I was level 7 and 6 I still did this. Albeit I only got one or two balls, the manager would come in, give them to me, I checked them and if they needed more air I would find myself giving them back to sort out, and A) never see them again B) they came back still too flat (embarrassing when you have to ask again or C) having to chase which, again is embarrassing. If I had a ball pump myself, everything is sorted there and then.
All in all - to get something done, right, and quickly, have a ball pump.
Each to their own though!