I also agree with Jock that if you intend to wear another clan's tartan that it would be the courteous (& proper) thing to do to ask permission (rather you receive a reply or not).
I tend to agree that it would be the courteous and proper thing to do, to ask permission. But if you receive no reply, is it still courteous and proper to go ahead and wear it anyway? In other words, why ask at all if the answer is irrelevant?