Yes, put the gauges on after you have made sure you understand what the test conditions must be....
You will lose almost no refrigerant...only that in the hose line..... and you need to have the top fitting a tiny bit loose until you have put some refrigerant into it... and then when you hear some escaping tighten it down... that is called purging the line... ( of air )...
" how often will I be using them "... sorta of a funny thought... so the possibility of not getting the correct readings perhaps the only time you will use them is comforting ?
LOL

They will probably work just fine...

Find Vstech's description of the criteria for test conditions...

__________________
1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html