Depends on how you define sensitive but read on http://about.ask.com.../about/privacy.shtml
Many different oppinions about toolbars, I happen to like Googles since it has possible useful functions. Each button can be moved as well so more a collection of buttons than a static pixel eating toolbar. Like Web developer toolbar for Firefox.
Look out for those amateur toolbars from Conduit
or rather build with their tools. Anyone can do that so there are 1000s available. They can contain whatever, even scripts. Easier to put in affiliate IDs and such tricks. I am sure it has been done already but if you want to make some money build one for porn or torrent/warez links, add affiiate links - custom search is there already. How Conduit make money, possibly it is shared with you. If you make a default toolbar it is harmless but as you will see from their site marketing ideas are welcomed there. Sign up and test. Marketing = need for stats which is why toolbar phone home to server when someone installs. There could be more but I don't think it has ever been proven anything evil is going on, just marketing
Conduit don't control what users add of content so who knows. How well they check each one I don't know. Most toolbars you might find interesting are probably from Conduit. You can see it from the homepage each toolbar has. There is a program you can buy for developing your own, don't remember name but could be solution to "risk" - if you must have toolbar.