Some answers from my experiences.
You don't know 'exactly' what the color will be, but if the fabrics coordinate online, they will generally coordinate in person even if they are slightly different from the monitor image. I never try to match something on the monitor with something I have in my hand. If I'm trying to match a fabric I already have, I shop in person.
As for the 'feel' (aka, the hand) of a fabric, you get to know what specific manufacturers fabrics feel like and if you shop those lines online, you'll generally be happy with what you get. Word spreads here pretty fast if something is thinner or looser than usual, and, of course, you can always ask. A lot of times the members who are store owners are gracious enough to let us know when a manufacturer has switched to a thinner weight, even if only for one or two of their lines, because they frequently see things before they hit the retail market. We're lucky to have them.
Personally, I never pre-shop at an LQS and then go buy what I was looking at online instead. I think that's really a stinky thing to do and I just plain won't do it. To each their own, but I couldn't live with myself.