Depends on the design of the speaker and crossover. Most speakers are designed to have the tweeter close to the mid, but dash locations and upper door can work well if done properly. Crossover slope matters as well, because ALL crossovers, whether active or passive, implement a phase shift to the drivers. Only crossovers that don't cause a phase shift are digital. Tweeters perform differently, and really you won't know what works best until you get them in your vehicle, b/c all vehicles have different acoustic properties. Kick panels are generally accepted as the best place to mount mids because the pathlength difference is the least.