Flowcharting a function or procedure is a good thing. For one, if your flowchart is too big, it's a candidate for refactoring, at the very least. (Too big means more than 7-10 box-thingies.)

The underlying question of "Should I design before I code?" should be answered as thus:

If you don't, you're dumb.

Unfortunately, if you get into the corporate world, you (more often than not) have managers who push deadlines over design, resulting in a cancerous blob of an application. *shrugs*

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We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.