Shade cloth over the top of my greenhouse, positive or negative?

I live in N. California zone 9b. I always wanted a greenhouse, which is a total indulgence because I have a very long growing season and don't need a greenhouse. My son and I built a small greenhouse (I already wish it was bigger).  Mostly I use it to grow my seedlings until I can plant them in the garden. Even in the spring, actually late winter it can get quite hot. It's already been 102. I can easily put shade cloth over the top.
You might be scratching your head thinking Jen this is a total no brainier. But... I didn't manage to get the shade cloth on last year. Those seedlings survived some very hot temperatures.  I had to water every day. I didn't want to plant the tomatoes and peppers until we had night time temps at or above 50 degrees. It was a strange cool spring for us, and I didn't plan until May. I know that is normal for lots of you, but pretty late for me. My tomatoes, peppers, and eggplant produced earlier  by a lot then ever before, and continued to produce through the the late fall. Once the warm weather finally came it was recorded breaking heat for consecutive days over 100.  
Here's my dilemma. Almost everyone I talked to in my area had a terrible garden year. Tomatoes didn't grow, or produce well.  I know some is luck. I do lots of things that could have contributed. I try to build healthy soil, over plant a very diverse plants. Pay attention to water, and don't use any chemicals, even organic.
So seeing the 102 temp. I thought I should get shade cloth, but then I began to wonder if maybe starting out there life growing in the extreme heat conditioned them to continue to grow and produce in weather that normally stunts growth and production.
I know it's unrealistic to expect a definite answer to my question, but I'm very interested in what you think. Should I put shade cloth on, or leave it off?
Thanks

I am in zone 6.  I have a shade cloth….that I have not yet used.

I live at 9,000 ft. in the Rockies, I keep a shade cloth on the greenhouse starting in about July and usually don't bother to take it down until the end of the season (I have it just over the apex of the hoops, so once autumn hits the sun comes in under the side). I feel that it doesn't change the temp in the greenhouse all that much, but we get lots of sun scald up here and it gives the plants a little respite from that.