Tomme Arthur's Dubbel recipe in "Brew Like a Monk" says to add raisins at the end of the boil. I have made that recipe a couple or three times and first time just threw chopped raisins in the last few minutes of the boil. The other time(s) I had researched some on how Tomme uses raisins, and ended up caramelizing the raisins (9 oz per 5 gal of beer) in a bit of wort in a hot metal frying pan, deglazed with maybe an ounce of brandy, and then pureed that in a blender and added to the kettle at knockout. Both ways made great dubbel.
It certainly won't hurt to have boiled the raisins the full boil - as previously stated, it's just that you won't get as much raisin flavor that way.