Great subject! As both Kenneth and D Max wrote, it is highly subjective and tied to your preference and equipment. To be able to vary the EI and adjusting for it during development is the main reason I started developing film myself.
As mentioned above, you need to develop a feel for the impact of these changes yourself. I did get some great results with IIlford FP4+ at ISO 200 as I wanted to get a very hard, film noire result which I like for cityscapes (http://monochrome.me.uk/blog/browse/tag/noteAE/1) . At other times I have shot FP4+ at 100 and 80 ISO. So it is not just a case of lowering the boxspeed at bit.
I mainly vary the EI based on lighting conditions: if the scene has little or no contrast, I lower the ISO setting on my camera a bit to overexpose 1/3 or 2/3 of a stop and if there is too much contrast on a sunny day in the middle of the afternoon, I go the other way. Shooting medium format with only 10 or 16 exposures on a film does help as it is easy to finish the film, note the development recommendations on the paper on the outside of the film and load a new film for different lighting situations.
If I know I want to stand develop the film, I usually shoot at a higher EI to avoid blowing out highlights because of this development method.
I do take the film I’m using into account, for example, ADOX CHS 25, my favourite film, cannot be overexposed and I always shoot it on boxspeed even when stand developing.