Should the gas be turned off before leaving on vacation?
Yes, turning off your gas will prevent issues if there is a gas leak. I never have but I do turn off my water in case of a leak and oil burner to prevent wasting fuel. Turning off gas will be a personal preference, just as turning off the water is.  You will need to consider what you have for appliances and how much hassle they will be to relight or if you need them to run (like a furnace during cold months to prevent pipe freezing) and whether turning it off will help you relax more while on vacation or not. Some people turn off everything, some don't do anything.  Like I said, it's a personal preference.