There really is no bad time to see the glaciers. You'll be in Alaska close to solstice, so the sun will be up most of the time. Some photographers like early morning or late evening light since it isn't so harsh, but "early morning" means 3-4am and "late evening" means 12-1am, so you'd have to forego sleep AND find an operator who will fly at those hours (unlikely). The direction the glacier faces is also an issue (usually west-ish, but variable), and the choice of glacier is up to the operator. If your tour operator flies small float planes, they can't get that close to most glaciers-- tidewater glaciers can be quite unstable-- so bring binoculars if you want to see things up close. I guess the bottom line is that any time to see the glaciers is fine as long as the sky is clear. If it's cloudy/foggy you won't see much regardless of when you go.

Oh yeah-- remember to bring good sunglasses if the weather is good. There can be a lot of glare from the ice and the water.