If you use 9 in 10 or 7 in 8 the you get a bit more honey in a full super. If you are a migratory bee keeper and bring supers home to extract then you are a bit more efficient.
The positive of being more easy to uncap is balanced by the negative is that you have to handle more honey through the cappings system.
To answer the original question, always 8 or 10 with new foundation.