Anti-foam agents like Fermcap-S during the boil may help reduce your chance of a boil over, but you're better off spending a little more for the 10 gallon kettle (weldless). It will more than pay for itself in terms of less worrying, less wasted wort, and less clean-up. Spend the little extra, if you can.
You need the extra room to handle boiling all the hot liquor beforehand (if doing AG), as well as all the initial (pre-boil) wort volume (which can be significant)--especially since it expands and boils.
"Double the volume" of your final "batch size" to get your kettle volume is a good "rule-of-thumb."
I do 10 gallon batches in 20 gallon kettle (with Fermcap-S in the boil)--no boilovers.