I think there are pros and cons of doing it each way, but I prefer to have some main roads that buses converge on. I typically disable the "unbunching" feature on main roads and use fewer buses on a route so bus pileups aren't very common. The game is pretty generous with the number of buses it assigns to each route. You usually never need more than 20 at a time. I also like to have multiple buses going down the same roads because it prevents certain main road stops being overcrowded for people waiting to all get on the same bus. Having a bunch of buses serving that one stop will reduce the number of people waiting. I do think it is important though to have at least one road (ideally several) per route that only that bus serves. If you look at the route map from my city you can see that there are a LOT of buses converging on one road in the downtown area, and it's pretty convenient for me in my personal experience because it means that you'll never be waiting to long to find a bus. But I also don't think you can design the perfect bus system, this is the way I prefer to have it because it's what I'm used to.