I'm of the opinion that the sporran completes the overall look of the outfit.

That being said, just like with any clothing or accessory, it can be overdone. For me, it all depends on my mood. If I want to be less conspicous (or at least as best you can in a kilt), there are some relatively plain sporrans out there. If I want to stand out a bit more, there are sporrans for that as well.

As far as covering up the tartan, I figure if you've got 8-9 yards af tartan wrapped around you, it doesn't hurt to cover up a bit of it.