I put all my trellises on the north sides of my boxes and grow my maters, cukes and squash on them.
Keski

What kind of squash ? Winter squash grow on trellis (for example acorn, butternut, spaghetti, delicata, etc). Summer squash do not trellis (straight or crookneck yellow squash, and zucchini). I learned that lesson the hard way and wasted valuable trellis real estate my first year. There are many good threads with info on how to stake summer squash so it still grows vertically - just not on your trellis. Here are just a few ...
http://faq.gardenweb.com/faq/lists/sqfoot/2005033057023592.html
http://forums.gardenweb.com/forums/load/sqfoot/msg0514444811976.html
http://forums.gardenweb.com/forums/load/sqfoot/msg0310241914233.html

Bush beans do not trellis (they only grow 1+ foot tall). Pole beans do go on the trellis. A ton of peas can be crammed in not much space ... you can plant them on both sides of the trellis about 1" or so away from the trellis on each side AND space them 1-2" apart (check pkg for your variety). So that means you could have 2 rows of peas with each containing 6 (2" spacing) to 12 peas (1" spacing) for a total of 12-24 pea plants along one linear foot of trellis. Some people even plant the peas closer than that with some almost touching each other and others no more than 1" apart. Check your package for plant height as there are some peas that do not need to be trellised.

Also, be sure that the tomatoes you are trellising are Indeterminate, which can grow 6 to 12 feet tall. Determinate tomatoes usually only grow to about 3-4 feet tall and can be staked other ways than using trellis space. For example, a tomato cage.