Drop tanks have been a very contentious addition to Traveller. There is no question that drop tanks are canon. Even GURPS Traveller mentions the Kinunir, as an example of the use of drop tanks on commercial vessels.
Their use has been hotly debated on the TML (Traveller Mailing List) and in other venues also. The reason for this is that full adoption of drop tanks changes the economic balance of trade in the game. J-6 ships can now effectively go 12 parsecs in a two week period, instead of a standard 6 parsecs with one week burned up traveling to and from a starport or gas giant to refuel. Over the course of a year, with reusable drop tanks, lighters to deliver full tanks to jump points and refueling stations above GG, the speed with which major merchant craft can travel effectively makes the Free Trader obsolete.
This does not even address the effects such craft will have on the XBoat network.
For this reason IMTU I've limited the effectiveness of drop tanks by not allowing them to be jettisoned prior to jump. They can be used to power the jump drive, but you have to take them along. When you come out on the other side you can ditch them if you want, or not. Whether this is done depends on why they are being used.
Why use them at all in that case, if they count against your displacement? To allow a ship to cross a stellar gap for a special expedition. To increase the range of a scout vessel or courier so they don't have to stop to refuel in a war zone. In other words for special missions. This pretty much limits them to IN, Scout and Megacorp fleet use.
I differentiate between Drop Tanks and Demountable Tanks because I think it adds texture. Demountable tanks are something a Far Trader might invest in to allow it to capture that highly profitable route just a single parsec past its normal range. Drop tanks are something the squadron commander might use to give him that extra edge in scouting out the enemy.
This is how I use them. IMTU VMMV. See Demountable Tanks for a more complete explaination of my drop tank philosophy.