If you have the funds for it, I'd say go ahead and buy the contacts. I always enjoy seeing cosplayers with the sharingan. Plus it'd be kinda irritating to have the mask over one eye all day, I'd think. Gives you a bit of a break?
But if you can't afford it, don't worry about it. It's also cool to be normal Kakashi. Bring along a hentai book and sit in a tree, lol. (I saw a Kakashi doing this at Anime North this year, it was great.)