That's up to you. In my book Leatherneck tartan is on a par with a club tie-- it shows that the wearer is (or was) a Marine. I suspect that some Marines may feel "protective" about the tartan, while others probably couldn't care less. My guess is that with the present expanded role of all branches of the US military, veterans may, in the future, take a more proprietary view of these tartans adopting the attitude that "If you didn't serve, you don't deserve to wear it." This, frankly, is my opinion-- one I can assure you isn't necessarily shared by all.

Given that there are many people out there who would like to show affection for a branch of service to which they have no direct connection (ie: they didn't serve) it would seem to me that some enterprising merchant ought to come up with a range of "Memorial" tartans. These would be similar to those associated with the various services (including police and fire), but wouldn't imply that the wearer had ever served in that branch. This would allow someone to commemorate the service of an ancestor, without inadvertently giving offense to a veteran.

So, should you wear Leatherneck tartan? It's really up to you.