As has been stated a couple of times above, I also don't wear flashes or garter ties with boots. If I'm dressing casual enough for 8" boots, I just wear boot socks and it looks great. I also wear a rather casual day sporran and a casual shirt, no buttons below the nipples (this is my way of saying "not a button down shirt").

Do this with a nice tartan kilt, and you get a really good, casual effect which looks natural because you aren't trying to hard, but you still look sharper than the guy wearing the UK with his 5" leather belt with the 6lbs of chrome stapled to it. Casual, but nice looking.