I mulch as soon as I transplant. I don't transplant until May 1st though so the soil is pretty warm by then.

As hot as it is already, yes!

However, I planted some extra early, Mar. 1st. I mulched 1/2 of them, left the others alone. We had 2 late frosts and the non-mulched survived, the mulched died!

My soil temps were 70 + but I guess the mulch kept the heat of the soil from saving them.