it is perfectly legal. the rule only says the initial contact point must be the base. it does not limit you on how you hit it.
thanks since i have been able to get pretty tight service with a slice service was kinda worried that it might be called illegal if i joined any competitions
make sure you do what the pros do. point the base squarely towards yourself/racket face so there is no ambiguity that you are contacting the base first.
Probably the more contentious issue than your slicing service would be the height of your service contact. Ensure that your contact is near the hip/waist area, your racquet head is not higher than the cone and you should be fine.
There are many different techniques, but all of them involve hitting the shuttle with an angled rather than a flat racket face. This makes the shuttle move in a different direction than the racket.
The major point of serving with a (slight!) slice is that it changes the trajectory of the shuttle. It drops faster -- you probably can see why that is desirable.
