In all narrowbodies, a lot of times, I've usually seen that it really doesn't matter! All passengers will often board board regardless of whther they're business class or not. Generally, the preboarding is announced so the parents with small kids and the disabled can board first. That's how I go, as I am hearing impaired. Yes, I've gotten the same kind of jealous looks from other passengers as they look at business class passengers, as they see that I can speak well and walk like anybody else, not being wheelchair-bound or wearing crutches, but that's often because they don't understand what a problem it is not being able to make out what the gate agent's saying in a crowded boarding lounge! Then it's general boarding time for the rest.
Widebodies are a different kettle of fish, however. What I've seen is that business and first class passengers will board first, then the economy class passengers will board by row sections, as the gate agents will announce. This is especially true for 747s of all types and DC-10s/MD-11s and L1011s. It's been about seven years since I last went on a widebody, but I still remember what they do. When my brother and I went to SYD from YVR, we had to connect in HNL, as is the usual thing for transpacific flights, we had to board a Qantas 747-200.
The 747 is such a big plane that at HNL and probably at other major airports, they not only have the econ passengers board by row section, but the 747 will sometimes have two separate jetways connected to it, one for the first class/ business class pax and one for the econ passengers - this was definitely the case in our boarding at HNL and probably (it's been a long time now) in SYD when we got on a 744 to CNS and for the return trip home.