Pricking out toms

I do prick out once the first true leaves appear, but also move things off the heat mat as soon as germination has started to stop them getting too leggy. It feels to me (no scientific evidence, just observation - assuming the memory is right!) that everything is a little slower to move on if pricked out any earlier.
But having had no germination of Gardeners' delight last year I sowed all of the packet this year and of course now have a pot full of seedlings. I'll do some now and leave some until they have true leaves and then see the difference.