Somewhere I read someone actually advocating cutting the gasket if it is too tight. Basically their argument was that the latex gaskets are only meant to stretch so much and if your neck is stretching it too much (or you need to use a can), you're screwing up its properties and shortening its life. They do put the little lines around the gasket for a reason I guess. Whether or not you want to brave it probably depends on just how unreasonably tight it is. I find after several days of use the gasket always becomes significantly looser, so a tight fit right out of the bag does not necessarily mean it needs to be trimmed.
As for how, I think most people lightly stretch it around something and use a razor blade. Fortunately I have a fairly skinny neck so never had to worry about it. Here's the Kokatat info, but I'm sure google will turn out all kinds of directions: http://www.kokatat.com/customer_service_faq.asp#10