The brakes are XT and the new rotors drag slightly. Will the pistons reset with a few heavy stops or is there something else I should do…?

Did you push the pistons back when you fitted the new discs? Also you’re meant to change pads (not saying that I ever do) when changing discs, as the previous pads will have a bit of a shape to them matched to the old disc

See if it’s still the same after a few proper rides.

Push the pistons back, check the caliper is centred and that both pistons are moving freely.

Push the pistons back, check the caliper is centred

+1 one, rotors differ. Don’t expect it to be centred, just because your last one was

Thanks guys. Took the advice and the front is sorted the back needs a little tweaking but will be sorted by the time everything is burnt in.