You may exile more cards than necessary for March of Otherworldly Light's first ability, but you can't reduce the mana it costs to less than {W} this way.

For example, say you wanted to choose a target with mana value 6. You need X to be at least 6. You could choose to cast March of Otherworldly Light by paying {6}{W}. You could also exile white cards from your hand to reduce that cost by {2} for each one. If you had a reason to, you could even choose a greater value for X.

The mana value of March of Otherworldly Light while it's on the stack is the value chosen for X plus 1, no matter how much mana you actually paid to cast it.