Believe it or not we did the Princess Alaska cruise (10 days out of San Fran) this past June. All I can say is It was Great!!!

and......... It was disappointing!!!!
Alaska cruise? I live there right???? Well Grandma (mine who just turned 80 and lost grandpa the previous year) wanted to do the cruise to Alaska with the entire family all 38 of us. (although I did ask if she'd change her mind and do something like mexico or the carribean).
Loved:
being with the entire family (no one else lives up here)
being waited on
beautiful scenery
seeing southeast Alaska (never been, it's too far from us)
weather was nice only 1 day of rain
some really neat excursions (pricey though)
and did I mention being with the family
Didn't Like:
Didn't have much for kids to do!!!!!!! (the major complaint)
the pools were only open for 3 days of the 10
the high cost of things
rough waters in North Pacific (some sea sickness)
so much more of Alaska to see
Stayed too long in ports w/ not much to see/do
stayed too short in ports w/ lots more to see/do
If you want to really see Alaska it's not the way to go, but if you want a brief overview of Southern coastal Alaska then it's good. But check very carefully with the cruise line you're thinking about to be sure it has the amenities, ports of call, and length you're looking for.
There is so much more diversity when you move farther North. The natives, the russian settlements, the artist communities, the fishing, the wildlife, the mountains (including Denali, or otherwise known as Mt McKinley), rivers, lakes, and the never ending sun (from end of may to end of July)
I would definitely recommend visiting in the summer time (July is the best month seeing the interior), unless you love

cold, snow, ice, and darkness (only about 4-5 hours of sun in the height of winter, where I'm at and I live south of Anchorage)
Actually to answer your question: they are limited to very few weeks of the year for cruising (it can be dangerous most times what with the ice bergs and such) so any time they are available is a good time.
Sorry I was long winded. If there is anything you'd like to know that I haven't already discussed

(like that was possible with the length of this post) just PM me I love to "talk" about my home!