I usually keep any FSD that gets a 50% or better. If I want to try for more than that, I would store the 50%+ unit and roll on a new FSD.
Of course, YMMV depending on how many ships you have/want.
Class 6 FSD is also used on the Corvette and T-9
After being a bit of a ship collector, I now have several Anaconds outfitted differently for specialized functions. Explorer, Passenger missions, All purpose, and another one to act as additional storage and as a shuttle to/from Engineers to do modifications.
HTH
In general, if you get a good roll on a module but want to try for more, store the good roll and start again with a fresh module until you have more than you need. After that when you have an excess of materials, make another pass on the weakest unit in your inventory. Doing it like this allows you to get a sizable fleet fully modded in a reasonable time frame.
You can also place some clean modules in storage prior to going to an Engineer and then transfer them there and swap out modules for more mods/visit.
I once had 100 FSD rolls banked and stored a few of almost every class before taking a Conda to Farseer. Transfer the modules you want to mod while your modding the first FSD. Once the first one is acceptable, swap to a clean FSD and so on. You can put any size on an Anaconda except Class 7 - You could do likewise with a Beluga or Cutter, but I already had those done so I took an Anaconda.
By the time I was done, my whole fleet had really nice FSD's