I leave mine covered until the seedlings touch the plastic covering that I use. Most of the time its when the first buds start to show.

Patrick

I raised over 24000 (twentyfour thousend) seedlings in the last 30 plus years. I never covered mine.
"NO NEED" to cover seedlings at all if you transplant them right away within 2 days of germination, or leave them where they are. Only if you transplant later on when there are many new roots and loose most of the soil arround the roots would I cover mine for about 1 to 2 weeks.
Read my article "Germination Explosion" on my articles page.
George Mander

Here is a link that might be useful: Roses of Excellence