As you mention, it varies from person to person. For riders that I travelled with on the east coast, most would get enough local cash to get from major center to major center. Of course you can usually get cash in smaller centers along the way but it's nice to be pleasantly surprised rather than have to rely on it. Plus, one never really has a sense of what the next city or town is like, it's just a dot on the map until you show up. In hindsight, it will seem like it never was really a problem and you didn't have to worry, but a bit of overplanning never hurt anyone.
As a backup, keeping a few hundred (~$500?) USD hidden away in the vehicle is probably not a bad idea. Might seem like a lot, but in a pinch someone in town will usually take USD (even if they will give you a terrible exchange rate) and when you are really in a spot, I've seen roughly that amount demanded to truck a broken down bike into town from a remote area, with payment demanded up front.
On the east coast, Sudan used to be the hardest spot to get any cash, and even Khartoum didn't have bank machines that would work for western bank cards, not sure if that is still the case. USD are better than Euros (Euros are better on the west coast). Once you get to Kenya and below, you will feel like you are on the tourist track and there will be plenty of more options for banking and other tourist services in general.