do you follow this so called rule??

what?!?! well I can see her logic if both the mono and damier patterns are clearly visible for everyone to see (eg. hanging a mono cles on a damier bag)... so in this case I think she is *somewhat* right. However, people never see your wallet, like who's going to be tall enough and peer down into a hopefully non-zipped bag to see what type of wallet you carry?!?! nuts I tell ya.... lol
 
I can see where the sales associate is coming from, but it's not like having Gucci monogram with your Classic Monogram Louis Vuitton bag - I think that is pushing it a little. I would wear the two Louis Vuitton lines together and do it with pride.
 
I don't like the mix of Damier Ebene and Monogram so I won't do it (it's a personal thing, like I won't mix hardware colors.) I do like the look of monogram with Damier Azur, Vernis or Denim though.

I won't let an SA dictate what I will and will not buy. I want to mix lines, I will. However, my SA is good, she really knows my taste and can usually lead me to looking at something that I might not consider.