But if the source of gasoline in the sample is the same as what is available at the pump, then you can more accurately match the standards to the sample. EPA method 8015 suggests that if you know the source of the contamination to use that for the calibration standards if possible.
We always went to the local stations and got a 40ml vial of each fuel(since most stations in town are served by the same vendor) and use that to weigh up a stock standard into methanol. We store both the source and diluted standards in a freezer at -18C. We also purchase a blended composite fuel standard from someone like Restek and compare the two sources, they should agree within the allowances of the methods used(+/-15% for EPA8015).
I like to replace the station gasoline each year, but stored in the freezer I have found each years new compares very well with last years old source, even after a few years it still holds up well.