The question of when and when not to tip can be difficult in a foreign country. In Uganda and Rwanda, it is customary to tip your driver/guide at the end of a safari or hike, as well as a cook or porter that accompanies you. A figure if roughly $5 per day would be a fair benchmark, though check this with your safari company in advance. I see no reason why you shouldn’t give a bigger or smaller tip based on the quality of service. It is not essential to tip the guides who take you around in national parks and other reserves, but it is recommended, and the money will be greatly appreciated by the recipient.
The thing to remember is that whoever you tip in USD will not get the sum of money you intend to give. The exchange of USD is not an exact science, the rate given depends on both the age of the note and the size of the note. The newest and biggest denomination note will attract the best rate. A $1 bill will attract an absolute rubbish rate no matter how many you have. A Rwandan / Ugandan will be happy to receive a tip in whatever currency you want to use but for day to day living they prefer their local currency. If you tip them with dollars the first thing they do is go to the forex to negotiate the best rate available, either that or try and sell them to back to you.
So at the end of the day if all you have is USD or GBP or Euro then use that currency but the best option for the recipient is the local currency (Uganda Shilling or Rwandan franc).
But please please please don't do what some people do, tip using foreign coins, particularly one pound or one euro coins as they have no value at all and yes some people do do it.
It is customary to tip for service in local bars and restaurants, though you may sometimes want to leave a tip (in fact, given the difficulty of finding change in Uganda), you may particularly be forced into doing this in some circumstances. A tip of 5% would be very acceptable and 10% generous. Generally any restaurant that caters primarily for tourists and to wealthy Ugandans/ Rwandese residents will automatically add a service charge to the bill, but since there’s no telling where that service charge ends up, it would be reasonable to reward good service with a cash tip.