Depending on where you live, expect to pay $400 or so. Also, since the V70 has an interference engine (meaning that valves could collide if the belt snaps) I would definitely not procrastinate too long on a timing belt change.
Injector cleaners work well and I have had good success with a product called BG 44K. However, I would not use something like this until you have logged some significant miles on the car (60,000-75,000). In the meantime, be sure to use quality fuels like Chevron that already have detergents like Techron mixed in. As your car ages, you'll find that a can of BG 44k or similar can help clean out carbon deposits and other contaminants that have built up over time.
All things considered, rotating your tires is beneficial in extending their service life - but by no means do you have to pay the dealer's labor rates to accomplish this. A local wheel/tire shop typically has lower overhead than a dealership and will charge less for this service. Alternately, you can elect to have this done free of charge when your car is in for a brake service and/or suspension service - really any service in which they must remove the wheels - since pulling the wheels on and off is part of what you are already paying for. Alternately, you can invest in a floor jack, jack stands and a torque wrench and do it yourself. The investment will pay off in the long run.