Really, that depends on what you're trying to do. If you're going for Uncle Ben's 4-Cola Topping Technique (Look at me, I already memorized the title of that thread!) then yeah, you can top for just 2-4 colas. I pretty much always do Uncle Ben's technique. Pay attention to what is mentioned about the seed branches, they're always small and puny, they won't make big colas like the first true node does.
However, if you want a bushy girl, with lots of bud sites, you can keep on topping each branch as they grow out. A good idea when doing it this way is to trim away any branches that get long and stretchy above at least one node so that they become at least two more branches. This will keep your girl nice and short while multiplying your bud sites.
When you're getting near ready to flower, try not to touch your girl for a week or two. This will make sure she is fully recovered from the stress of topping and can devote all her energy towards making flowers ASAP.