What you are noticing is the difference between your determinate and indeterminate plants. Check the plant tags or your seed packets, and you'll probably discover that your determinate varieties are the ones that don't need staking. Determinate tomatoes can usually stand on their own without additional support. The plants are small enough to stay upright when they start producing fruit. That said, keep an eye on your determinate plants; you may have to give the prolific producers some support. Something as simple as tying the plant to a wooden stake or poles can do the job. Indeterminate tomatoes have a tendency to become lanky and rangy, especially the cherry types, so plan on providing support. Buy ready-made cages, or fashion your own from 6-foot lengths of 6-inch-mesh concrete-reinforcing wire. Stake the cages to keep them upright on gusty days.