You can use either way. Common rail wiring ( using only one insulator ) is eisier and cheaper since you only need to run one power feed wire. In other words, one rail has power all the time and the other rail is insulated and has the power feed wire coming from some type of on and off switch. But, make sure you always insulate the same rail or you will cause a short in the wiring. Run one wire and test the wiring, that way if you do cause a short you know right where to look. If your track has a lot of curves it can get confusing !
Two rail wiring has both rails insulated and you run power feed wires to both rails, meaning you need more wire and insulators.
By the way, is this dc or dcc, and are you wiring your whole layout this way? If your using dcc, you really don't need block wiring, except your engine service tracks, that I know of.
Loren