Some people do prefer to blend the essential oils with a carrier, but this can make the bath slippery. I have known one company to sell a bath dispersant or milk can be used.
As an aromatherapist, I add my drops (usually about 6-8) straight to the bath, but remember to do it just before you are about to get in, as it can evaporate quite quickly.
There are also some oils which can cause a skin reaction when used in hot water, so always start with less drops.
Lesley