I use a flexible arctic grade hose, suitable for propane, between the tank and regulator. (I have these custom-made by a reputable local company.) POL fitting on the tank end and NPT (National Pipe Thread Taper) on the regulator end. (Up-to-date two-stage regulator ... regulators do have expiration dates and you should not use an expired regulator.)
I use NPT threaded black iron pipe exclusively for my distribution networks. The tapered NPT threads create a liquid-tight connection when tightened sufficiently. Careful extra tightening can lead to a nearly gas-tight connection. (Gas as in gaseous, not liquid gasoline.). Over tightening can crack the pipe and fitting, rendering the connection useless.
Until now, I used Teflon thread compound or tape to ensure the NPT connections remain gas-tight. As soon as I find a source for the compound/tape that StingrayL82 mentioned, I will switch to that. (Purchased on or before the next time I work on a propane system.)
The NPT threads and thread compound/tape is a 'belt-and-suspenders' approach to insuring gas-tight connections. Flare nut fittings are inherently gas-tight, as long as you ensure the flares are well-formed, kept clean, and adequately tightened. (NO thread compound/tape in or on the flare.)
I use brass flare nut fittings at the distribution network terminals. Final connection to propane devices is via flared copper tubing or manufactured metal flex hoses suitable for propane (LPG) or natural gas.
All connections are tested, under pressure, with a dish soap/water solution prior to being put into service. (Special focus on connections within the building.)
My cabin was originally set up for off-grid use. In addition to a non-electric propane cookstove, we had a propane refrigerator, several propane light fixtures, and a propane heater. The propane light fixtures are still there and used during power outages. The propane refrigerator was replaced with an electric one when the electric grid reached the edge of our property. The propane cookstove was more than my wife and I needed and took up precious space in our 16' x 16' cabin. So, it was removed and replaced with a countertop convection oven, a microwave, and electric hotplates.
The system terminations for the propane refrigerator and cookstove were capped so we can revert to off-grid configuration if we want. We have discussed installing a propane cooktop but what we have works for us.
The cabin's propane system was inspected and subjected to an extended leak-down test, with no discernible drop in system pressure. (The inspector/tester was impressed with my installation, which is why the company tried to hire me.)
