Let me answer by paraphrasing Benjammin: none of them, or all of them !
Your current seals are likely all made of the same material... either they will handle the switch from whatever you're currently running to bio (and/or ULSD) or they won't. If they do, no problem. If they don't... they will all eventually go. You'll replace one, and then the internal pump pressure will find the next one that's about to go.
IMHO, if you're planning to run B100 and you're worried about your seals... pull your pump, buy a 30 dollar complete seal kit, and do 'em all. Otherwise, you'll find yourself doing one, and then another, and then another... if they start to leak at all. Some of them require you to pull the pump anyway... and your odds of success are probably better doing them on a clean workbench.
Just my CDN $0.02...
Vince