Depends on your relationship with your letter writer and how you plan the LOR with them. For instance, a hastily written e-mail to a professor who taught your Intro Bio class of hundreds two years ago will likely NOT be a good letter.
Conversely, if you take the time to get to know your writers (office hours, smaller classes/multiple classes, out-of-class interaction, for instance) then you can at least have a better shot from a hastily written e-mail.
What you should do is give your LOR writers resources on you: your PS (or an outline of it), your resume, grades, and a cover letter or face-to-face discussion will help them get an idea of your goals, what you've done and so forth. If anything, doing this will remind them of you, which makes it all the easier from their end to write glowing words.
See it from the writer's perspective: it's difficult to write unique things about somebody when you don't know much about them.. so give them as many resources and angles to consider. And above all, do this ahead of time. People are busy and may not get back to you in time, so plan your LORs months in advance so the gears start turning.