Are these plants you are planning on buying mail order or locally? Around here, perennials start showing up locally about early April. It's usually safe to plant them in late April - the temperature of concern is about 28°F.

planning on buying locally - have a couple good nurseries (Kuperas & Heaven Hill) that I've used that sell things for my area, work in my soil, & have enough selection

If the plants are fully hardy for your zone, there is no real need to be overly concerned about last frost date. Too late or too early still runs the risk of frozen soils, but if the soil is workable and the plant hardy, you can plant. Frost dates are generally used as benchmarks more with regards to tender perennials/annuals. Perennials with more succulent foliage, like hosta, can be damaged (not killed) by cold if they are fully leafed out then but will releaf rapidly.

Although my soils don't freeze to any significance here, we do experience lots of frost, some rather late season and some quite severe. It doesn't bother the perennials at all. I've even planted a full perennial border in mid-December right before a 2 week cold snap into the low 20's - the plants never even blinked.

In northern IL/southern WI we try to get perennials in the ground in April. Only tender annuals wait until after the last frost date (about May 10). We used to be in zone 4 here, now who knows. Anyway, if the temps get really brutal--say in the 20's, we just cover them overnight.

Thanks to all - now I can plan for spring w/o confusion!
All the Best,
Theresa