I think that emailing the professors before you decide to apply to the school where they teach is very crucial to the Ph.D. application. Doing so should greatly increase the chance of getting admitted. First, after getting their replies, you will see which professors are still active in research, which professors are planning to shift their research interests, which professors are switching to other schools, and, most importantly, which professors are considering taking a Ph.D. student in the following academic year.
Second, if you don't get a professor's reply, this implies that he might not be interested in you or too busy to emailing you, or that he is not willing to open a dialogue to you before he begins to review your application. It is no harm as long as you keep your first email short, concise, and polite. You can still apply to this school, but it's difficult to predict your chance of getting in.
Third, you should have a better understanding of the professors' current research from their replies and thus be able to rule out some whose field of interest does not interest you. This could narrow down your "shopping list" and help you find accurate matches. Sometimes a professor will send you his recent research papers. This is a good sign. If you are interested in his research, you could easily find out some research questions in the papers he sends you. If you can not find any questions in his papers, then he may not be the one you would like to work under for four or more years.
I emailed 13 professors in different schools in North America and Europe and got 10 replies. From my experiences, the chance that the schools with higher ranking will reply is lower. It, however, really depends on each professor, the school policy, and your "touting strategy", I think. I once got an email back from a nice professor at MIT but no reply from a professor at the school out of top 100. So, this really depends.
From the "touting process", I narrowed down my "shopping list" to 9 schools in North America. 7 of them replied to me (I dicided not to apply to MIT). From their replies I selected 4 from the 7 schools as my "high priority", which doesn't mean that I want to get in them most but just means that they are highly interested in me. I exchanged many emails with the professors in thses schools to talk about my research interest. I think I am on their candidate list already.