Having the right tools to install, trouble-shoot, repair ect. is a must. We need these tools to do our job accurately and efficiently so we can walk away from the job confidently, knowing we did the job correctly. I have taken a few calls in the past, where it was necessary to question the tools ability to give us the correct information. These calls were from veteran techs who pride themselves on doing the job right. These guys really did "check everything", except one little thing, the tools themselves. They invested hours and even days on the job only to discover that their tools were actually letting them down. 

When it comes to "goofy" readings, take a moment to consider that your meter may not be reading correctly. Invest in back up equipment (maybe not every tool but your most critical ones) for those just in case moments. An inexpensive back up is ok. Take care of them ALL, test and check their performance regularly, keep them maintained (batteries, calibration ect). And most importantly, don't be afraid to invest in yourself!