Put a drop of vinegar on each rock - if the surface fizzes - don't use that rock. It means it will slowly dissolve and affect your water.
If it doesn't react to vinegar - simple dry out the rock thoroughly in the sun - this will kill off any bacteria on said rock (well, most of it anyway), and give it a quick scrub to clear the rock of algae or moss/dirt etc.
Other than than that - you're pretty much good to go.
I think boiling is a bit of overklill myself (just my opinion) although it does guarantee no snails or other hitchikers, but a good scrub and a bit of sunlight should take care of that anyway.
Take great care in how you place them in the tank though, you really don't want them falling over in there - it's tempting to make walls out of them and stack them to look good and make caves etc - but if they do topple it can be a real disaster - so play it on the safe side.