While wearing garters is a must, as hose will tend to work their way south without them, wearing garter tabs or flashes is certainly just a way to add a bit more color or visual interest.

Then there's the 'traditional garter tie', which is what I reccomend, where you'll have the ends of the garters themselves sticking out from beneath the hose cuffs.

Personally, I find myself going with no visible flashes/garter ends most of the time, unless I'm really dressing up. I just use a pair of garters that my mom crocheted for me years ago, and rap them around, tuck in the ends, and turn the hose cuff all the way over the whole deal. Nothing wrong with that, especially if you want a very casual, understated look.

With the sgian dubh, I think most folks use it as a fashion accessory, though many are regular knife carriers and have had sgians made that are useable knives (myself included). Personally, I wouldn't wear a sgian without a useable blade, and I often times wear no sgian at all, favoring a pen knife in my sporran, or a larger folder clipped inside the waistband of my kilt.

As to laws, I would strongly reccomend that you pick up a copy of Knife Laws of the 50 States, if you're very conserned about the matter.

Generally speaking though, as long as you're not in an airport or government building, and as long as you've not been caught in the act of committing a crime, it seems that the fuzz will leave you be.