It depends on the type of plant. Tomatoes can be planted right up to the first set of leaves and they will produce roots from the stem but for pumpkins and most other vegies, just keep the soil level the same as it is in the
seedlings mix. Otherwise you risk collar rot and the death of the plant. If the pumpkins are a bit leggy, give them a bit of support when you plant them out. I use a small pot with the base cut out over the
seedlings. It also can be tilted to give a bit of shade protection until the plants harden off.