Industry standards allow for 5-7% waste in cuttings and defects. Measuring Rooms for Hardwood Flooring unless you have odd angled rooms or off grade material, this should provide enough material. If you want to have extra wood in case of damage or plank replacement it is a good idea to purchase an extra box so that you know the finishes match and the profile matches.
If you are buying first quality material which is well manufactured and graded, you simply do not need to order excessive volumes of material. The exception to this rule would be unless you are going to do a lot of further hand selection prior to installation.