I do not like wheel spacers (generally used to change the bolt pattern so that you can bolt on some killer wheels you like off of a brand of car that won't normally fit your car; example a set of Mopar Magnum or Ford Shelby wheels). The reason I do not like wheel spacers are two fold; but both reasons are based upon the fact that a wheel spacer changes the track (width) of the car's suspension.
First thing is the added stress placed upon wheel bearings and ball joints because you have increased the length of the lever arm that applies an additional torque on the parts.
The second thing is your steering handling and alignment will be affected by the added track, as you will now magnify any movement about the pivot point of the front end. This will make your steering twitchy wanting to dart one way or the other following every crack in the road.
As to having mismatched tire sizes hot rods have used big and littles since the fifties as a bigger rear tire has been required to obtain enough traction to accelerate the car without slipping the tire. Today's rubber compounds have all but eliminated that issue, but the tradition continues because everybody else is doing it (hearding instinct).