How far to go when Validating a SQL Service Pack install?
The ideal situation is to have a server that, in terms of OS and other software, exactly matches the production environment, upon which you can perform the update first. That way you know everything that is required.

This would preferably be a VM that you could snapshot, so if you encounter and fix a problem you can revert to the snapshot and start again with your updated procedure. Repeat this until the upgrade procedure works and you know the reboot requirements and such, then plan to repeat the process in production.

One of your development/testing VMs might be ideal for this if you have them (i.e. if your dev/test/release process isn't just "smash code together and throw directly into production"!). This way you are essentially treating the service pack the same as you would one of your own bug fix or feature releases meaning you can perform a full regression test on your application after the service pack is applied to the test environment (to make sure MS haven't introduced any bugs or changes to undefined behaviour that your application depends upon - or that they haven't fixed a bug that your code is depending on!).

Obviously this "ideal" could be more time consuming than other options...