^ as above. I do mine more frequently but I don't sharpen as much. I just prefer that when my saw touches wood, it bites really well every time, so mine gets a tiny bit of filing after every tankful.
If I do some cutting (like I did a couple of days ago) and don't finish a full tank, I still give the blade a swipe with the file, I just don't press as hard or as many times. You can see and feel the teeth of the saw when it's ultra-sharp - and a sharp saw is an efficient saw.
I used to neglect my chain and the saw would not cut straight or quickly. It would rub the wood more than cut it, so it would heat the wood and the chain, causing the chain to stretch even faster. Keeping the chain in prime condition means not only do you enjoy a better cut, but everything lasts longer.
The key is not to file away too much, and never use a machine unless the chain's actually blunt. Rakes must be filed at the same time as the teeth are sharpened - but you don't have to remove a lot from the rake.
The most important part to sharpening the chain is ensuring that the cutters are all the same sharpness and size, or you'll get vibration from the saw, particularly when cutting hard timbers. You should also ONLY file from the inside of the tooth to the outside, and I even withdraw the file to move it backwards for the next 'swipe' rather than allow the chance of the file to drag on the tooth.
Here's a Youtube video showing the details of the correct way to sharpen a chainsaw. They pay attention to everything that you need to bear in mind to have a GREAT saw blade.
[YT]https://www.youtube.com/watch?v=EWcckW3ghFg[/YT]
And here's a YT vid of someone using the exact same sharpening tool that I use, although they're not doing it entirely correctly - they're dragging the file backwards in the tooth.
[YT]https://www.youtube.com/watch?v=2txewgs6sEk[/YT]
I've a forest full of ironbark about 15 mins away, and I've tried taking a new bow saw to it - the bow saw is about as effective as a child's plastic saw on the wood. Forget a hatchet, and even a super sharp axe struggles to get through it. If your saw is not super sharp, you leave black marks on the wood - but if the saw has been maintained properly, it slices through the wood nicely. And the wood's fantastic in a campfire or my slow combustion heater - it lasts for ages!