Ruth Ann:

"Nice to have" - depends on the intended use and what you're trying to find out by having a sensor.

Oxygen sensors for enclosed spaces will warn you there's a low oxygen concentration issue.  I've seen these systems in enclosed spaces that don't have a lot of activity in them.  Generally speaking, if you're in a space that has a catastrophic failure of a LN2 Dewar, you know it happens and it's time to leave Dodge City.

There's a serious downside of placing sensors and monitors all over the place where there really aren't needed:  Maintenance.  The "best if used by" date that's on the package is a suggestion.  It was derived by placing the sensor in a perfect environment to measure its lifetime.  Texas, or anywhere else for that matter, is not the perfect ambient environment.  Elston's rule on sensors:  Change them twice as often as what is stated on the package.  Three times if you work in hot, humid environment.  Humidity is the enemy of most sensors.

Good luck

Harry