If you go for wider then 12" be careful about the actual size of the tire when choosing a spacer.
For exemple my Nitto Trail Grappler size is 35X12.5R20 but on the Nitto web site they say that the actual size is 35.04 high and 13.39 wide (I didn't measure it myself but just to be sure I got myself 10mm - 3/8in Hub Centric Spacer)
For comparison Goodyear Wrangler MT/R actual size is 34.8 high and 12.5 wide and Toyo Open Country M/T actual size is 35 high and 13.0 wide.
Normaly a 12" wide tire shouldn't need a spacer but people recommend a 1/4 spacer for a 12.5" tire because they assume that it's only half an inch wider (or 1/4 inch closer inside) but since all 12.5" aren't the same you might want to make sure you know the actual tire size and how much spacer you will need!
just my 2 cents