If you're buying UV filters for protection purposes and you intend using them regularly, I suggest buying them in the right size for the lens. Mucking around with step up filters can be a hassle. In many cases, they preclude the use of the right lens hood, and they get stuck to the step up filter very easily and are a hassle to remove (at least mine do - but I use cheaper step up filters).
For filters that you might use irregulalry, such as infrared, polarizers, ND and graduated ND filters, rather than buy a lot of lower quality filters, just buy one quality 77mm filter and use step up rings. You are saving yourself some money and getting better results at the same time. 77mm is a size that works well with most Canon lenses as many lenses have a 77mm filter thread anyway (and there are very few - if any? - with a bigger filter thread).