fins front and fins back - its one of the great divides between surfers!
But with a softboard, its definitely worth doing fins front because softboards will bend.
If you strap it on with ratchet straps or similar, just be careful not to pull too tight - plenty of rails dented this way. There are some tricks you can find on the internet
eg - apart from the fins facing back... but notice where the straps go, down the side of the board rather than over the side rails (if you have side rails)
http://www.its.caltech.edu/~surfer/cont ... ctions.pdf