The Pokalyzer (
www.pokalyzer.com) gives you exact percentages, not random samples, by simply dealing out every possible flop with the remaining cards. You can setup up to 10 hands preflop and optional flop cards. It also has a table of every starting hand against every other starting hand. The table was built by dealing over 700 billion hands and recording the results. You can download a trial version free, registration is $30.
I should point out that I am involved with the Pokalyzer but in the interest of fairness I would mention that Steve Brecher has also released a very good (and free) command-line based programme called Hold'ems Showdown. Can't remember the link, but you should be able to find it using Google or the like.
In terms of accuracy, simulation programs which deal, say, 1000 or 100,000 hands usually come pretty close. After all, if you are about 58% to win and find out that the actual chance of you winning is 57.33%, you'd still make the same decision.