Just asking is great, I do it every time it is possible also to take pictures of properties of people (e.g. a classic car or a pet of someone) and usually people not only agree but they are kind of honored that someone noticed them. Even people one would usually think to be more problematic such as beggars, graffiti artists, skaters or bikers are usually more receptive than one would think once one is clear about what is the planned use of the pictures. Even after a no usually people are not as annoyed as when one tries to picture them without permission. It seems to me that it is easier to ask if one shows some pictures and "waist" some time explaining exactly what is the project one is working on.
GLF