My thought on this is that when you reach a point when you want or need to perform a full system re-installation, then you should add definitely add SP3.
Bear in mind that you will almost certainly have installed all the security fixes listed in Bushfiva's link to your SP2 installation, and possibly some others also.
Most reports of problems with SP3 that I have seen appear to be when installing to an existing SP2 configuration, so be sure that you really are prepared (with backups etc.) for the possibility of some pain.
SD