I think it really depends. If many plants are growing together in a flat, but they are just starting to get their first true leaves (and don't appear really crowded) you might just leave them be for a bit. Transplanting now, however, might be easier on the plant while they are small if you intend to transplant to move then to individual pots.

If they are already in individual pots you probably will be able to tell when they start getting root bound to move them up to the next size pot. If they get too rootbound their growth will 'stall'. Sort of a big deal because if you are like me you want them as big as possible when they go into the garden.

I started mine in larger foam cups but they are now 5 inches high and getting rootbound, so it's time to move them up to bigger pots to keep them growing good.

Glen

If you're talking about planting them outside, it's way too early. I plant mine out in mid to late May - they spend the day outside and come in at night before that. If they are exposed to temperatures below 10C their growth will slow.

If you're talking about bigger pots, then Glen's advice is good.

BP