From what I have read you still need spacer unless you change wheel/rims. As for the warranty from what I have read the warranty is only void if the modifications you do, lift, etc. cause a problem.
My plan is to go with lift/leveling first. Nothing bigger then 2.5" and then look at tires. I want to avoid the domino effect of modifications, lift and tires that leed to new gearing, new wheel etc. for a vehicle that I love but will be doing 95% street driving.