A single speed chainline is important to get straight, since you have only one and it works best that way.

but with derailleurs , especially a triple , lets say a 9 speed cassette,
only the middle chainring will be in line with the 5th cog in back, the one in the center of the cluster.
so the middle chainring will line up with 4th or 6th chainring, It's not an issue . or as said above ,
no big deal get a different BB theyre only $20 these days whole cartridge kit.