USDA planting zones are based on average lowest winter temperature, which is a good indicator of what perennial plants and trees/shrubs can or can't survive.
For planting out in spring, I would be more concerned about last average frost date and the night time lows. As long as temps are staying mostly in the 50's or above you can plant tomatoes. They can take mid 40's and occasional low 40's with no set back. You CAN sow squash seeds at the same timeframe, but it can take a while to sprout depending on soil temperature. I've had some success with covering the spot where the seeds are with cut-off plastic bottle cloche.
It's possible to plant out tomatoes earlier than this, but you will need to protect the plants if cold or frost threatens. Successive daytime highs in the 40's seems to completely stunt them.
When the forecast lows are mostly in the upper 50's with no chance of falling down into the 40's -- usualy about a week later -- I plant peppers and also squash transplants IF I started them inside (which is very rarely). Squash seeds sown at this time comes up more satisfyingly.
To accomplish this, I start hardening them off about a week earlier. So tomatoes go out by day and in by night (or get extra protection but stay outside) while night lows are mostly in the 40's with occasional dip in the 30's, and when the tomatoes get planted, peppers (and squash) start the hardening off process.