I don't think it matters much. I'm not a fan of instant notifications (avg. importance of my email is too low to justify the amount of distraction), but beyond that, checking frequency would be pretty low on my list of email productivity improvements.
I check mine once or twice a day; most of my email is pre-sorted correctly by using a bunch of filters and around a dozen of addresses with different uses and priorities. I don't think I could work with a global, unsegmented email inbox; I saw a friend use his years ago and it still terrifies me, even with his relatively low inflow (30-50/day).
What would be high on your list, out of interest?
How frequently do you think you *should* check email? You can also say how frequently you do in comments.
Personally I'm sold on thinking you should check it around once a day, not necessarily without fail. That increases focus on both email and non-email, and minimises getting sucked into distractions. But some people I know disagree. Some believe in getting notifications whenever a new email comes in.
For anyone who'd like to check email less often and uses GMail, I recommend using http://inboxpause.com/ and this full screen compose link: https://mail.google.com/mail/u/0/?ui=2&view=cm&fs=1&tf=1&shva=1
Edited to add: I'd recommend everyone at least try checking only once a day, at least for a few days, to see if you find it more productive and/or relaxing. That'd be a big enough win to make experimenting worthwhile.