I'll add a few comments here:

Take into consideration, virtually ALL tartans have a SPECIFIC meaning and "baggage" attatched to them. For example, the Royal Stewart is "officially" the tartan of the royal family, but has been opened to all her subjects and those that are thier decendants and those who ally with her. The Black Watch is the tartan of a specific old military unit (the "government" unit during the Jacobite years). It has been used by many, many units since then and is now seen as a "general" tartan. Many I know DO NOT wear these two tartans for the Royal and Government associations they bear. Likewise, Caledonia is a very,very old pattern and was used in the 18th and probably the 17th century. It is used to represent Scotland as a whole today. Each and almost every tartan has similar associations.

Sometimes, a tartan has more than one association. One of these is the same Black Watch mentioned above. A good article on this is:
http://albanach.org/tartanname.htm
One on Black Watch is here:
http://albanach.org/blackwatch.htm

While one is free to wear (almost) any tartan, each has one or more meaning and to wear that tartan ties the wearer to the meaning. In some cases, it is to make a claim that one has no right to (such as the Balmoral tartan, which is understood to be limited to the royal family itself-> so to wear it bears the claim of being royalty). I analogize it to sports colors. In both sides of the Atlantic, football (in US-> NFL or college and in Europe-> football/soccer) provides a good example. Each team has a set of colors, a livery associated with THAT team. While free to wear whatever colors you want, to wear these colors is to associate yourself with that team. So:
Anywhere in the US, to wear:
orange and a white paw-> Clemson University
Green and Yellow-> Green Bay Packers (NFL) or Oakland A's (MLB)
Purple and yellow-> Minnisota Vikings
Black, Light Blue, and silver or white-> Carolina Panthers
(You get the idea)
This is accentuated with logos added to the colors.

A few weeks ago (the day of the Atlanta Falcons/Carolina Panthers football game), I wore a Falcons jersey and a Black Watch kilt on the battery in Charleston, South Carolina (Panther country). It seemed more people noticed the jersey more than the kilt!!! I stood out as a Falcons fan and got pro and con comments on the team (with only 1 mention of the kilt at all!!), no negative "skirt comments" ("Trust a Falcons fan to wear a skirt"-> never heard). The colors tied me to the team (and, thereby, all the other Falcons fans)

A Tartan is similar.
MacLeod ties to the MacLeods
MacNeil to the MacNeils
Wallace to Wallace
USA Bicenntenial/St Andrews-> US citizen/supporter
Carolina-> North and South Carolina
(again-> you get the idea)

Therefore, while one is "free" to wear whatever tartan he/she wishes, one is identified with the meaning of that tartan, and they become a representative of that tartan. It is best to know what that tartan is, what it means, and be able to discuss it at least somewhat. Imagine:
A person who wears a Packers jersey and is asked "How about those Packers?"
Person-> "The who?"
Questioner-> "The team your wearing."
Person-> "Oh,I just saw it and liked the colors."
Most likely this will result in a look of disgust, if not downright anger.

Moral: Wear what you like, but know what you wear and have a good reason for it. The best reason-> My ????? was a Mac????

PS-> I am an Atlanta Falcons fan, as that is where I was born and raised. By second favorite team (by only a small margin)-> Panthers, for I have lived here since the team can into being.