I do this, as do a number of people with whom I play. However, a number of other people with whom I play tend to make lists of what they want their characters to be able to do ... and things they don't want to affect them ... and then build around that conceptual list. The former is akin to writing up a backstory extemporaneously and then adding powers based on what one wrote ... while the latter is akin to deciding one wants to build a brick and then listing out the aspects of that brick followed by layering in the backstory afterward." Both approaches work. Both are fair game.