Unfortunately, 9 times out of 10, unless spectators see a standard red or blue bicycle backed card, they immediately "think" in their minds that it could be a "magic" or "trick" deck.
I disagree. I use nothing but custom decks and have for years and have never had someone give any indication of suspicion of it being a trick deck. The only time, literally one time, I have ever had someone ask if I was using a trick deck was when I was using a deck of red backed bikes.
I personally believe that the folks who get questioned about custom cards are projecting the idea themselves that the cards are fishy. Either that, or performers are assuming people will think the cards are fishy when the spectators are thinking no such thing.
I have also noticed magicians tend to pre-emptively try to disprove that cards are gimmicked, thus putting the idea into the heads of the spectators who wouldn't have thought of it otherwise.
I also have a very easy argument to back this up: Go to Walgreens and look at their playing card section. Standard Bikes, these shell-back designs, black backed bikes, zombie cards, Duck Dynasty Cards, etc. Go to Target - Bicycle Custom series, different zombies, angry birds, star wars, etc. Go to WalMart - Bicycle Nautica, Leaf Back, Tattoo Club, Expert Back, etc. Go to the liquor store - Jack Daniels, bud light, etc.
Custom cards are everywhere. People are just as likely to have a deck of Coke cards as they are to have Standards. We're the ones that focus on this - not them.