well... it's not quite that cut and dry folks. but yes, in most applications, it is better to have them seperated.
generally speaking, if you can have most of your sound coming from a single point, it will sound better. and in a car, this can be accomplished with either kicks or time alignment. barring those two, increasing the pathlength on tweeters does tend to help.
02 Mustang GT... Tuned by Nelsons. Low 12's, anyone?

.....______
___|______\_____
|/-\_________/-\_|
.\_/...............\_/