Sure you can wear a tweed jacket without a tie. Sometimes it looks good; soetimes not. It really depends. I'll eschew a necktie occasionally for dinner at home, when I'm kilted.

Here is an old pic of the late Lord Lovat, sans necktie. I'd hate to be the one to call him out on it!

http://www.gstatic.com/hostedimg/a0dfee3991c6777a_large