That will work. As to whether or not it will give you any battery issues varies device to device. In general, pulling down email more often will use more battery than pulling down email less often.
However, since you're not connected to a data source most of the time, it's hard to say whether it will use more battery or not. Some devices will recognize that they do not have a data connection and will stop trying to sync email, whereas some will continue to try even without a data connection, which will hit the battery pretty hard.
This can be something you can experiment with. Set it up for 5 minutes all the time and check your battery level at the end of the day, then set it back for a longer time period and check your battery level at the end of the day (with basically the same usage other than the sync setting). If there's a significant difference, you can infer that your device is checking for email and continuing to try even when you don't have data.
Alternatively, since you said you usually only check your email when you're off peak, you can set it to check more often during the off peak times and less often during peak times. It's kinda the opposite of the way it was intended to work, but that should work well for you.
