A bell is a good idea if you're riding where people are walking, or on paths where it may be difficult to pass slower cyclists. For some reason - I've never figured out why - pedestrians seem to be less alarmed by a bell than by someone calling out. Maybe they interpret a shout as being more aggressive, or something. Anyway, go with what works, is my view, so I have a bell on my touring bike, which is the one I'm likeliest to use in those circumstances.