There is a certain problem with German taxes. If you book:
FRA - NYC (for business)
NYC - SFO (pleasure)
SFO - NRT (for business)
NRT - FRA (for business)
the tax authorities shall/should/will conclude that part of your travel was leisure and will devide the full ticket price into a leisure portion and a business portion. I agree to Rambuster that you CAN declare everything as business. You should find a good reason for a business visit (based on the above itinery) for SFO, too. Easy with SFO, but might become difficult with some exotic places. My (in general BKG or CAI travel agent issues me in invoice mentioning the major stops (or what I understand to be the major stops) of my trip. Never had any problems - I never understand a strange island to be a major stop.