I plant them and peppers closer than recommend as they self shade each other and the ground below them.

The first year I cut off the suckers-little leaves growing from the forks of the branches. Then I covered all the plants with shade cloth. They suffered.

Last year I planted a bunch of tomatoes too close, did not cut off any suckers and just left them alone. They did well. As "NoiseBeam" mentioned the tomatoes grew tall and created their own shade and microenvironment.

This year, I am planting tomatoes with more space between them. I use the Weave method of staking the branches. Again, will allow them to get tall and overgrown to create their own environment.

"Last year I planted a bunch of tomatoes too close, did not cut off any suckers and just left them alone. They did well."

The most successful tomato season I ever had was done this way. I concluded that tomatoes love company and do best when left alone to grow however they want to. If I had the same space available to me now that I had then, this is what I'd do. Btw, they were in full sun (southern exposure) w/no shade cloth.