The RNGs don't work on a percentage, the games have the percentage built in. If you're playing single zero (European) roulette there is a 2.7% edge in favour of the house. With double zero (American) roulette this increases to over 5% towards the house. Playing for any period of time means that the house will eventually come out on top.
You're quite correct, when playing for free, there's less emotion involved and people are prepared to take greater risks. They also will be less inclined to come out of the game when the funds are getting lower as it's not 'real' money and this is often where the 'winning streak' kicks off. Of course, if you're playing with you own cash, it's probable that you'll have left the table whilst you still had enough cash for a pint and a taxi home!
Iain.