The N9 user guide (page 102) says "Always charge the battery fully." to improve battery life.

Whereas this is what LifeHacker has to say:
Similarly, lithium-ion batteries don't need to be charged all the way to 100%. In fact, they'd prefer not to be—so the 40%-80% rule you heard is a good guideline

Who do I believe?

Everybody lies (c) Gregory House

i have a N900 (4 years) and a N9 (2 years) both always charged to full and waited less than 10-15% to put on charge, both batteries are almost new (N900 lost 3-4% of total capacity and N9 1%)
a couple of times happened i had to charge them before the battery was low or remove the charger before full charge, in both cases battery life decreased really faster
i also have a laptop, charge period wasn't regular and many times i didn't disconnect the charger; now i don't have a battery anymore: after 7 years the capacity became 0mAh

Thanks! Do you leave your phone on the charger even when the battery is at a 100%?

Hm.. I charge it every night, stays on charger till the morning. And sometimes connect to pc at work during day. I dont know, smartphone batteries never last longer than one day for me. Unless i'm not using it

You're quoting out of context. The full text is:

"If it seems you're always looking for a charger, there are steps you can take to reduce the power consumption of your phone, and keep talking for longer.

• Always charge the battery fully."

Of course if you don't charge fully you'll run out of power sooner. Note that this advice has nothing to do with ensuring the durability of the battery.

However since batteries only have a limited amount of charcing cycles before degrading it makes sense to make full use of each cycle by using up the charge as much as circumstances allow and then charge fully.

Which is correct but should not be an issue. Overcharging can kill li-ion batteries and durability generally suffers when charging to the limit. However the battery management of the device should take care of that (for example by not making full use of the capacity in the first place by not charging to the absolute limit) and it is therefore generally a non-issue on the comsumer side. Provided battery management is working properly there should be no side effects from charging fully.

Thanks. Do you know if N9 comes equipped with battery management?

Yes it does. The easiest way to spot this is to check charging current at 50% charge and at 90%. Current gets lower and lower to make battery last longer.

yep, i think is worst charge over 25-30 than remove before 100% but you have to consider that N9 battery level decrease faster when under 40-50% (it's a bug) so it's better leave it charging till full

If you're talking about the sudden drop to 4% from battery levels as high as 50% I too thought that this is a bug. I tried all the cures that were offered here (restarting battery monitor, etc.) but none of them really solved the problem. In my case replacing the battery immediately and permanently solved the problem. So I think this "bug" is simply a bad battery. The fact that those drops regularly happened when the phone was working under medium or heavy load (and therefore drew significant power from the battery) seems to support this finding.

However I nevertheless think that there is a bug involved: For even if you have a "bad" battery, the phones battery management will still declare the battery as "good" when you check in settings -> phone -> batterey -> battery status at the bottom of the page. This status never ever changed despite heavy use of the phone and the battery probably being defective from the very start (my problems started less that half a year after using the phone.

So if you experience large and sudden drops in remaining battery-capacity it's time to change the battery.