Just remember when you buy a "new" mac the oldest version of OS X you can downgrade to is the one it came with. If you system came pre-bundled with Lion then installing Snow Leopard is not an option. The prevention to do this is defined in the Hardware Abstraction Layer or HAL.

Some of the earlier i3,i5, and i7 iMacs (mid 2011) shipped with 10.6.6 and can be downgraded from Lion if you installed it but the newer releases (late 2011) will not downgrade below 10.7

also when your solution involves virtualizing an OS you may have compatibilty problems and performance issues. In other words booting to an dedicated windows partition will be much faster than running a virtualized session alongside Mac OS since your CPU is now running two or more OS's instead of one. You may not notice this will spreadhseets and web browsing but with games and 3D you will see a difference.