As you can see, Col. Mac, it depends on where you "grew up." I first wore garters and flashes in Boy Scouts (stateside), so I wear 'em directly athwartships.

Those with early exposure to military practice, probably wear them more toward the front than I.

'Tis no a uniform, and 'tis no a costume[1], so wear them where you think looks good.

[1] .. except of course, when it is a uniform or a costume. The Pipe Major, NCO, or costumer will let you know if this applies!