I tend to find that the default Joomla! Tool Tips tend to be a little busy and obstrusive. In most of the sites I build I am more and more focusing on a more uncluttered user interface in my design work.
This is just here a litle aide de memoire for how to just get rid of them completely.
In your css for the site:
.tooltip-inner{ display:none; }
This definately works in Joomla! 3, I'll let you know if I can confirm it for Joomla! 2.5