When transplanting how deep do they go

Some of my seedlings look a bit leggy, both flower and veg, I know that tomatoes can be planted up to the seed leaf but can you do the same with other seedlings, or are there some plants that you can't plant deeper than the the root ball?

I was watching Charles Dowding's propagation videos this very morning, and he says most seedlings can be planted deeply. The exception, if I remember correctly, was lettuce, as they have leaves low on the stem. I think if the stem is bare, it should be fine as long as the soil isn't waterlogged.

Edit: yes, it's lettuce that can't be planted deep. This was the video i was watching - the bit about planting out is near the end, around the 19 minute mark: