I've read all on the Inside passage and the surrounding areas. We were thinking of visiting the area late September or early October; or alternately late November early December.
Is this possible / practical / wise ?

After the first part of September, a lot of tourist places close, and you're getting into much colder weather. I don't know what other Fodorites think, but I wouldn't even consider November or December because of heavy snowfalls.
We went late May and it was a wonderful time to go. I've also heard good things about going early June.

It depends WHAT you are going for???? AND WHERE? There are no cruises, but Sitka is great in early Nov with their whalefest. I greatly prefer March for Fairbanks- Ice Championships, aurora, dog sledding, but NONE of the "tourist" stuff open- which will be the same in October. Do more research and determine the above questions.

I went to the inside passage in July and the weather was fantastic. Got lucky with the rain except one day in Juneau when we got Dumped on. Ketchican gets 12 feet of rain per year mostly not in the summer it think. I'll give you a hint which will get you in good with the locals. NEVER, NEVER, NEVER complain that its raining. They take offense to it when a tourist whines agout rain. S.E. Alaska is a rain forest after all and that's what makes it so beautiful.

I recommend September as the latest. I went in July. It was lovely!

Mmmm These are challenging comments and are really making me think about the timing of our visit. The September October timeline is best for us , but we can look at all of July and the first two weeks of August as an alternative. But wont it be terribly crowded at that time???

You don't have to worry about crowds in Alaska there is so much space to spread out in.