One simple rule: do what's most cost-effecitve, if it's more cost-effective to not void it (eg. voiding the warranty would run you more money than is worth) then don't, if it's more cost-effecitive to void it (eg., water damage in all situations).
If you have a blanket warranty like AppleCare try really hard not to void it (water damage!), with programs like that you can get a new unit for the tiniest things, so that works out well. But if you have a limited warranty on say, a phone, and it needs a new antenna, and the company will charge you a ton to fix it, you might be able to do it yourself for cheaper, even though it involves voiding the warranty.
For the murky areas simply ask yourself if you can live with it and/or if you can find a workaround.