You will probably find that the 0.8mm bit becomes your most used size. Are you using a small drill press or drilling by hand? The smaller drill bits tend to break very easy. You will find opinions differ in this subject, too. Rule of thumb is that you want to have the least number of different sizes per board so that you are not having to change the bits often. Thus you might use a different size for a particular component from one board to the other. The rules are pretty flexible for hobbyist use. In the circuit board industry, the board layout person decides which sizes are used. This is given to the board house in a NC drill file. The sizes are usually already determined inside the board layout program when you choose a part from the library. If your board layout program generates NC drill files or Excellon files, it also makes what is called a tool file. This tool file lists the sizes that you are expected to use. This comes from the part package in the board layout program's library.
If you want to get as close as possible to the actual diameter of the leads on the part, you can look at the data sheet and plan to drill the hole just slightly larger than this. However, a tight fit is not necessary.
We use numerical sizes where I live. My most used size is a #70 or (.028"). This for ICs and common components such as resistors and caps.
I hope this is somewhat helpful. ;D
-MP
