Is it better to top up fuel at half a tank or to refuel when almost on reserve?

Is it better in the long run to top up fuel at half tank on a regular basis or to refuel when almost on reserve?

My car isn't reliable and keeps refusing to start. Can I get my money back?

I got my car - a 2015 Mercedes A-Class - at the end of January. Twice it’s been recovered from my drive as it wouldn’t start. The dealer I got it from is saying it’s a software update that needed doing...