This is an interesting one.
I use what is easiest. For casual and dress pants the fly is long enough and the pants are loose enough that the fly is convenient. However, it can be a challenge to use the fly on my low rise jeans, so I typically just undo everything. For example, these Rufskin jeans have a pretty tiny fly, and its just too much of a struggle. Hell, in these jeans its easier to just pop it over the waistband, ha ha.
Jack