Personally I wouldn't think wearing decent, clean trainers an issue at all assuming your in work wear such as your trousers and top with your logo. As long as the overall appearence is clean, smart and professional. However I would not want to canvass in anything less smart than that as would you buy from someone in tracksuit bottoms? The same as I would think wearing a suit a little over the top. (Just my opinion)
I too walk about in work boots, trousers and top when leafletting or canvassing and have recently changed to wearing clean trainers and it has not made any difference. The fact that your in a uniform top and decent trousers means that your already smarter than alot of the competition (in most places).
Tom