Battery - do they last longer if recharged when empty? I'm about to buy a Samsung Galaxy Ace (it's not much expensive and I think it's a pretty good phone), however everyone has been complaining with the battery life but admitting that isn't bad compared to all the other smartphones.

I read about how to save battery with tips/tricks, and I'll certainly do it; but I'd like to know whether a battery life would increase recharging it only when completely empty (some old phones used to gain life expectancy from that, I don't know if things are changed now)
Modern phones use Li-ion and Li-poly batteries. So the battery life is  not affected by whether you charge them at 0% or 90%. 
It only has a long term effect. Batteries degrade with every charge cycle. A cycle is usually a recharge from below 50%. So charging while the battery capacity is still relatively high has a long term effect of less degradation. 

The other frequent pseudoscientific habit that has no roots is a technique many call "formatting", where you leave your battery plugged in for 16 hours or 3 * 8 hours with discharge in between or any other baseless claim by anyone on any forum or phone shop. If they don't have scientific evidence don't believe them.