I always check to IE6 I don't believe that there is a large enough % of people still using IE5 to warrant you checking it, however it may still be useful.
Latest browser statistics is this: IE7 - 26.5% IE6 - 27.3% IE5 - 0.7% FF - 39.8% So I think that you only have to go back to IE6 unless you feel that you really need those last 0.7%. Source: http ://www.w3schools.com/browsers/browsers_stats.asp
We still have IE6 at work - as I'm sure many people with an overworked tech support staff or low budget do. My favorite is the sites that have a popup when a browser is IE6 that says that your browser is arcane and unstable and you need to upgrade. But I think you are safe if you don't go back any earlier.