Rule of thumb is less than 255 width tire go 1.5", 265-275 go with 1.25" and 1" with 285-295 and have a little poke. This is in most cases if ur running typical "mustang" wheels.
+1
Unless you have no plans on changing the wheels, spacers would be a waste of $ since you'd probably get ones with different offset and would need new spacers or no spacers at all.