not sure if there is a general idea. It very much depends what kind of pants, what kind of shirt, what your style is, and where you are wearing this getup
My general rule is that sometimes something fitted is good, but not all the time, otherwise you look too vain. Also remember, chicks can usually tell you're a hardbody whether or not you are wearing something fitted.
Also, don't confuse tight with "not-the-right-size" either. baby-ts look good on chicks, not on guys.
I beleive it looks alot better if your pants are a comfortable fit and the shirt is tight fit...I wouldn't wear a baggier shirt then pants...Unless it was a suit ...i'd stick with a tighter under shirt..
I'll usually wear medium baggie pants and a medium sized shirt that looks a little small (I'm starting to fit my shirts a little more .... getting a bit more buff). Everything balances out though.
rule of thumb for you male fashionistas', take it from the ladies who know never to mix bold eyes and lips, either one or the other, thus NEVER a straight baggy look, or skintight = overkill.