i have a few soccer balls and they are all a bit deflated.. im wondering is it better to keep them that way till i use them or should they always be pumped .to make them last longer??

Do any of them have holes in them? Because you might want to get the holes fixed (if there are holes I mean).

no no.. no holes.. its just over time they start to deflaated by themselves

^Yeah I know, but I got a football a few years ago that I only used for a bit, and then when I wanted to inflate it later, it kept losing air so I put it on water and found that there was actually a hole in it
Anyways, I was always told that you're supposed to make sure your football isn't over or under pressure (check to see if you have a label that tells you the recommended air pressure.
I also heard that some people release some of the air of their really good match footballs because it reduces how much stress is put on the stitching, and then they just inflate it the next time they're going to use it..

put a bit of saliva onto your finger and then put it into the hole of your ball where you put the pump in. if the bubles in the saliva get bigger, it means your ball is losing air. if not and it is a really shiny and new ball, it is probably your imagination

but should the balls always be pumped or should they be left to deflat??

^ As mentioned above:

Check your ball for the label, it should say the suggested air pressure that you should keep it at..

I'm a coach at a local school. I try to keep them inflated over the break. I believe it helps them retain their shape. This is just my opinon. i'm not sure as to what you're actually supposed to do.