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You could use a bit of loctite blue on the fastener's thread if you want. Normally I would tighten until snug then about a 1/8 of a turn more. All sealing is done by the O rings. Its a normal practice to replace the O rings at every AC service regardless of the condition of old rings. For this purpose you should have an assortment of O rings (get a kit) for AC works. This is better than ordering individual O rings according to its parts number and economical in the long run. Using Nylog blue (for R134a) at the O rings also helps in sealing the connection. No teflon tape should be used on any fastener's thread and in fact on any fastener of the car. Let only plumbers use them.

A less commonly mentioned GOOD practice is to keep pushing the AC pipes (with O rings on it) onto the compressor port/accumulator port/any AC connection ALL the while you're tightening the fasteners. This to ensure no skewing of the mating surfaces while tightening.


For O rings set:
https://www.shopcenturyautoair.com/store/p36/European_Import_O-Ring_Kit_CP3009.html

For Nylog blue:
https://www.ebay.com/itm/Refrigeration-Technologies-RT201B-Nylog-Gasket-Thread-Sealant-RT-B-BLUE/153101721030?epid=2254671118&hash=item23a592ddc6:g:w2oAAOSw1qtbTJbl


Amarin.