You really don't *have* to repot tomatoes just to get more roots... although it is a fairly space-saving way of doing it, since they don't occupy as much area under the lights until they are larger and repotted.
Other options include:
start in larger pots, don't do anything fancy, they will still grow ok when planted outside, especially if you plant them "up to their armpits" when you put them out.
or
start in larger pots but with 1/2 depth of soil, gradually hill up as they get taller (same as you would do with potatoes), they will grow some extra roots that way and do a bit better IME than the previously-mentioned strategy.
Honestly, unless you are in a race to get the first tomatoes in the neighborhood, or have a short growing season, I do not think it is especially vital to repot them, they really do grow okay without it.
That said, I start mine in flats or cell-paks and repot them, b/c I *like* repotting and am always short of space under the lights.
JME, good luck, have fun,
Pat