I suppose it largely depends on how you introduce it into the system.
For example FoundationTech has it's bulk in scripts/FoundationTech.py the script that loads FoundationTech in scripts/Custom/Autoload/ and it's standard techs in scripts/ftb/Techs and second party techs (techs not created for FTB, but while development of FoundationTech), and any other tech, in scripts/Custom/Techs/
Creating a little library module with your common functions is fine, and all your specific scripts would then use that library. Modularity is also improved here, so if someone says they only want scripts A, B and C from your mod, but not D, E and F, then with the proper modularity they could just copy the common part, and scripts A, B and C.
You mentioned QBA, in the QBautostart directory there is a lib directory, put your common scripts in there (or in a subdirectory, don't forget the __init__.py file!).