It's hard to say just about anything isn't a risk if javascript is on. As far as I know, there is no good way to tell where an exploit might exist. That said, if you are really concerned about highlighting text on a page without risk, my recommendations would be to either a) turn off scripts such that onclick events don't occur, b) view source and copy the text directly out of the source or c) if the text is not available in the source, use a tool like firebug which can inspect the DOM and pull the text directly out of the DOM.
In general no-script on a site you don't trust is just good security to begin with though as there have been exploits that don't even require direct interaction with the page beyond simply loading it.