I have to share my amazement at learning hands-on that seed planted early does not necessarily beat seed planted on time. The rationale for seed-planting guidelines is so painfully obvious when you yourself observe the large % of seeds that do not sprout with cool evenings and cold ground.
My advice is to definitely wait for warmth. Your seeds then magically sprout instead of rot, and produce volumes of plants instead of only around 30% of what you expect.
I suppose the opposite view is that seeds are cheap and if you get some early plants, it’s nice to have early vegetables. I guess you would then have to weigh the value of your time to the value of the earlier harvest. I can say it was fairly easy for me to re-seed the bare areas of my planted rows with fresh seed.
All I know is that next year, I’m not going to bother trying to out-perform my planting zone. =)
(Update: See photo – I originally planted 7 seeds in this cucumber mound, of which only 1 sprouted. After warmer weather arrived, I added 4 seeds… which you see ALL grew!)