Depends on how hardy the seedlings are... we start in the basement, move to a greenhouse. cool weather crops like broccoli lettuce etc need to harden off before going into the ground. most flower seedlings need to stay warmer and inside, as do warm weather vegies like tomatoes, corn.
the greenhouse gets very warm even when it's still cool outside, so you need to monitor and make sure things don't get either too cold at night or too hot during the day, either of which will kill things. we usually start to move things to the greenhouse in march. may still frost at night but not hard enough to kill anything. you need to water plants in a greenhouse much more often than you'd think... tiny pots, warm temps. in spring if it gets sunny we open the greenhouse lid to let out the heat.
"If the world were a logical place, men would ride side saddle." Rita Mae Brown