If the leader has been exposed to light for a long time, then it will no longer show maximum black. True solarization (less density for more exposure) is rare with modern film, under normal exposure conditions, but it can still happen to the leader of a 35mm cassette left exposed to brightish light for a long time.(posted 8425 days ago)
If you're referring to transparency film, then looking at the margin, or any other unexposed area, is a part of the assessment of processing quality.