do you mean residual curl?
If a roll film like 35mm film has been wound on the reel a long time it may want to curl up after processing, particularly if it has not been left hanging to dry with a weight ( a few clothes pegs etc.) clipped to the bottom. That explains end to end curl.
The across the film curl - what that comes from, I am not sure, but yes, it exists. Some films, after being left in plastic negative filing pages squashed into a binder for a while seem to flatten out. Otherwise the enlarger negative carrier generally flattens the negs as they get seated prior to exposure.
You don't mention it, but you are washing? Otherwise the hypo (sodium thiosulfate crystals) from the fixer would show once the film was dry. I have not tried to dry an unwashed film, but that could result in stiffness.
With time, you are likely going to see discussion of rapid fixers. They fix faster than what you are using, however what you are using is a great way to start. The 5-10 minutes guideline works like this. Take a bit if film - say the leader that you clipped off of the film that you just wrestled onto the reel (It does get easier with practice). Dangle it in the fixer in a cup. You can do this test with the lights on. The film clip will first go opaque. It will become clear. Note how long it takes to do this. Then fix your films for twice as long. With time, as the fixer is re-used, the amount of silver tied up in the fixer increases, and the ability of the fixer to clear the films a s quickly diminishes. Do the above 'clearing test' after say 5 rolls, then 10 rolls. That will teach you when you should be fixing longer. When the clip takes twice as long to clear as it did when fresh, then I call the fixer exhausted. I mix a fresh batch, and give the exhaused stuff to a local photo processing shop, who then on-sell it for a small amount to a silver recycling company. Otherwsie, label it, and turn it in at a hazardous waste facility. My local dump takes stuff like this, paints, used motor oil, etc. for free, while I pay to dispose of 'simple' garbage. Used fixer should not be idly poured down the drain.