Whether or not a tartan is "official" does indeed matter -- but only as much as you want it to.

While I certainly don't think anyone should be derided for choosing to wear a fashion tartan, I also don't think anyone should be derided for caring whether their tartan is official or not.

Just to give one example, several past cheifs of the Clan Campbell have made the point of stating that the Black Watch tartan is the official tartan for the clan. There are other approved clan tartans, such as Campbell of Cawdor, Campbell of Louden, and Campbell of Breadalbabe. But they have specifically pointed out that the so-called "Campbell of Argyll" and Dress Campbell are not proper clan tartans and should only be considered fashion or fancy tartans. And the "red Campbell" tartan? No such thing, as far as the cheifs are concerned.

Now, if you happen to like the Campbell of Argyll tartan and want to wear it despite what the cheif says, no one is going to fault you for it.

On the other hand, if by wearing your tartan you are attempting to show your pride and allegiance to your clan, it may be very important to you what the cheif of that clan declares the true clan tartan to be. The fact that the cheif has specifically said that this is not regarded as a clan tartan will be important in your decision.