Tink,
There is some truth to the tv horticulturists comment, BUT, only SOME truth. What you want to go light on until fruiting begins is nitrogen. You can fertilize with phosphate and potash or with any low nitrogen fertilizer. Too much nitrogen early on promotes heavy leaf growth ( not entirely a bad thing) but may cause fruiting to begin later. Once you begin to see tomatoes begin to ripen, a dose of nitrogen (In my opinion but not proven) makes the resulting tomatoes sweeter, just don't overdo or you will "burn" the leaves. I use a level tablespoon of ammonium nitrate (Yes, the explosive stuff) at the base of each plant every three weeks during fruiting season. I did not invent this regimen, I acquired it from my dad, who grew tomatoes for 70+ years.