When you vent a mass spec, moisture enters the system. Even turning off the pumps with the intention of maintaining vacuum allows moisture to enter the system. Moisture goes away slowly, so you have to leave an instrument pumping several hours before you can use it if it has been opened to the atmosphere. And, prolonged exposure to moisture harms the detector. So, pumps off only for instrument maintenance and scheduled power outages.
Tuning adjusts for drift in the electronics. For qualitative work, small changes in limit of detection, mass resolution, mass accuracy, and relative ion ratios may not be a problem. For quantitative work, you want to keep these more tightly controlled. At least run a tune check fairly frequently. Monitor mass resolution, mass accuracy and the intensity of masses. Personally, I recommend at least a tune check once a week. And, a leak check daily when you are using the system. The best check on the tune is a low level standard, checking the signal to noise of the analyte peak. Remember that when the tune check fails for a check standard fails, all samples are suspect back to the last good run. How many runs can you afford to reject based on failing QC checks? (Also bracket samples sets with QC samples so that you know that the instrument was good when you finish a set. Better than no QC and then a problem a week or two later when checking out the system. Was it an issue of the instrument standing, or where there bad results published?
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