I track - I used to use an app on my phone, but I wasn't paying for the app, and it wouldn't output the files so when I got a new phone I lost my database which pissed me off.
So I figured out a better method. Google doc form - accessible on my phone. Automatically enters a date in a spreadsheet, asks for 3 things.
Click submit and it puts the entry in the data sheet. Since I don't enter odometer it doesn't really matter if I do it every single time, but obviously if you want to track your overall fuel expense you should enter every fuel up.
Why I like it - you can easily do formulas to figure things out like your mileage - or your costs to fuel up, or your cost/mile.
This can automatically be done, or you can for instance sum up months and compare. It will show you when winter fuel comes in, or if you do your snowtires etc. I also have occasionally figured out my tire pressures were low based on reduction in mileage over a few tanks.