Lokelani- I know the dilemma and have been growing loads of perennials/biennuals from seed. I still have loads outside on a table. I don't have a greenhouse but have two very successful coldframes.
I've come to my own conclusion on this- if your plants have a good healthy root system and are fairly large plants I would plant them out this month. For me there is something uncomfortable about overwintering strong, large perennial plants. My thinking is that should be in the ground developing a good healthy root system for next year. It does depend of course on how big your plants are and where you live.
I've planted out my T&M 36 perennials that I got in May and the slugs haven't touched them. Might have to keep an eye out when they start coming through next year
Although not a popular or pc choice- you could always do a one dose of slug pellets to protect them and allow the leaves to develop so that they feed the plant ready for next year.