Easier to prep with no parts bolted on I think. Can you access the chain stays with the BB installed? I's also easier to hit the downtube from the BB shell as well. Spraying in the cable holes makes hitting the HT joint rather hard.
Pretty sure Ritchey must prep their tubes pretty well as they are triple butted. Still extra prep won't do any harm. What color bar tape did you go with? Yellow sounds like a good scheme and it does make you more visible on the road.