I don't know where you got that information, but it is false.
Running different slopes is fine. It is in fact required to achieve the best sound quality. Each driver will work optimally at different slopes.
I am far from an expert on the subject, but you may want to back up your statement with a bit more than "you're wrong." Stating that not only is it okay, but that it's required to achieve the best sound quality is subjective at best. I bet there are a lot of guys both here and competing who have been missing out on "lots of SQ" by running the same slopes between drivers; you should definitely enlighten us. Each driver may very well work optimally at different slopes individually, but from what I understand, slopes essentially impact phase. If you have a mid's LPF on a -12db/oct slope and are crossing the tweeters in at a -18db/oct slope, they would be 90* out of phase, correct? This can cause cancellation issues, etc.
I don't mind being publicly corrected, and if I am wrong, I apologize for providing bad information to the OP. But if you're going to call me out, provide the correct detailed information that contradicts my statements so we can all learn something, rather than making some arbitrary comment based on something you learned while pursuing the major under your username.
Maybe Chad or MVM can step in and set the record straight.