You don't have to . One of the benefits of "push" email. Although, there may be some occasional delays, depending on whether you are on BES (Blackberry Enterprise Server, or a corporate email) or on BIS (Blackberry Internet Server, personal email) and on what area you are in. For the most part, most emails arrive nearly instantly.
Personally, I have 7 email accounts setup on my BB... 3 msn accounts (1 actual MSN.com, 2 @live.com), 2 gmail accounts, 1 yahoo account, and the blackberry email). Generally, the emails will hit my BB 10-320 seconds prior to hitting the inbox per perspective email account. And I am using BIS. For example, my work emails will arrive on my BB roughly 2-5 minutes before it hits my work laptop, unless I force a refresh on my work PC. Which is quite strange since the emails actually arrive to my companies email server then get forwarded to another account, and then get "pushed" to my BB.