I would say that ultimately it is a matter of taste. If you like milk with your green tea then have at, jus know that you're weird.
That being said, here are some general guidelines.
1) Don't put both milk and lemon in your tea.
2) Lemon will go with just about any tea. While I think that it would overpower a nice sencha or a white tea, it won't hurt it.
3) Use Milk just with black teas. I'd avoid using it in a first flush darjeeling, and It will probably taste best in something malty like an assam or a dian hong, while lemon might taste better in a lighter black tea like ceylon.