I get a fair number of requests to review papers for journals and conferences, as well as grant applications. This is part of my job, and I am generally happy to oblige. However, I expect that you will at least ask me whether I am able to complete the review within the specified time before assigning it to me. Most journals and conferences I have dealt with follow this minimal standard of politeness, and in such cases, I usually say yes if I feel competent to do the review.
However, emailed or snail-mailed review requests, especially from sources I have never worked with before, that impose a deadline without requesting initial confirmation (e.g. by return email or web form) of my willingness to review will be accepted or ignored at my discretion.