Hydrometer readings

When is the best time to get your OG reading? I was told at the local beer shop that you can't get an accurate reading if you only do a 3g wort to prevent boil over and then add the 2g later. This would makes sense since not all the water is accounted for I suppose. So in that case, could you do a 3g wort, then right at cool down add 2g straight to the wort to = 5g, then take your reading as it's chilling?

Here's what i do, I chill my 3g wort, add to fermenter, top off with 2g bottled water, shake the you-know-what out of the whole thing, then take my hydrometer reading and pitch the yeast.

As TJD pointed out, your LHBS employee was half-right. You can't get an accurate OG reading from a partial boil unless you thoroughly mix the wort and the top off water. You're really going to have to shake the **** out of that fermenter to get it all mixed properly, so don't panic if your OG is off. It's pretty hard to screw the pooch on that using an extract kit.