Beware: if your registrar of choice goes bust within those 10 years, you probably lose your money.
There's also another risk: if you decide after 2 years you're not happy with your registrar, tough luck, you've already paid for 10 years.
The only time it becomes a problem is if your new registrar doesn't support N+1 year registration - your 8-year example means you need to find someplace that supports 9- or 10-year regs.