I like to only to get them to develope more roots.
What I mean is I pot them up to the topmost true leaves. Pinching off all the lower leaves and planting them deeper. The stem will then grow more roots. In the long run, you'll get a bigger plant.

I've tested this out with some done this way and some done planting at the initial soil level. It really did make a big difference.

This is also really easy if they are winter sown one plant per 2 liter bottle, you can pinch and add more soil without the repotting.

Thanks. I think I read one of your posts about doing that somewhere else so when I've repotted I've tried to put them deeper and cover them up to the leaves. I wasn't sure about whether the beginner leaves counted or just the true leaves so I was cautious. Now I'll plant them even deeper.

Caryl