Trying to get in touch with the original installer is always the first and best option, I was talking about the times when that is just not possible for a multitude of reasons. I've heard everything from 'I lost his number' to 'I can't call him because he's in rehab and doesn't have access to a phone'. One time a client even told me that he wanted to 'make sure that he (original installer) can't access the system, I don't want him spying on me - I don't like the way he was looking at my wife'. Another client wanted me to 'change everything, I don't want any of the old stuff' because she had just gone through a nasty divorce and all the tech was chosen by her ex and it reminded her of him and needed it gone (she must've made off with quite a good chunk of his money because she went all out).
Now just know here that I dont just come in and think I know it all and rip everything out, replacing it with my gear with no regard for the existing equipment. I'm a CI by day (and night sometimes - actually most nights, as I'm a sucker for a good problem that needs solving). I have a heavy IT background, hence the quotes when I said 'IT Guy' but no matter what your background is, I think we can all agree that when touching someone else's gear its always best to check, or if possible contact the original installer to see how/if what you'll be doing will impact the existing setup.
In the cases where you can't, either use kid gloves and try to work around the existing setup, or be thorough and make sure you cover all the bases. And rule number one always is 'if it ain't broke, don't fix it!'