If she cooks your dinner, washes your clothes and does the dishes.....I would let that slide.
I put fuel in my wife's car all the time when she tells me it's low. Mainly because I know That I will get a receipt and I know it will be the right fuel. I also go a few extra miles and save a lot of money doing so.....most people just cruise into the first gas station they see and fill 'er up.
We have wildly different prices on the same gas from the same bulk tanks at different stations.....so saving up to 20 or even 30 cents per litre (Quart for you people who measure in Washing Machines) makes a difference. Especially so because I also fill my spare bottles with diesel and premium.

Self-serve stations here are a lot cheaper than 'full serve' - that just means there is a jockey at the register inside....they don't come running out to check tyre pressures like in the '50's.