The tomatoes initially grow upward, then gradually fall over and the main stalk grows straight down, so almost all of the force is down and the roots are being pulled down into the pot, not up or to the side. When growing tomatoes, I was instructed to only use a couple pots on a stack due to the plant size, and to only put two plants per pot since the roots need a lot of room (I did sneak a couple habanero plants in with the tomatoes as a gift for the squirrels).
Haven't had any issues with breakage on the tomatoes (I do lose some pepper branches, but I think the squirrels may cause some of those). The spot in the picture where you can see a branch broken off was almost certainly done by me. I have to get out there just about every day and remove the sickly leaves and branches. I'm trying varieties which claim to be resistant to this, that, and the other virus, but so far I just have to keep them trimmed up.
The top of the highest pot is just under five feet, FWIW.