Modern derailleurs apply very little chain tension in an effort to minimize drag. At rest the both the upper an lower loops will sag. I'm not talking slack, jut normal sag under the chain's weight (look at any clothesline when no clothes are hung).

However, whenever you ride and apply the lest force to the pedal, the upper loop will straighten.

One problem on bikes with small inner rings is the height of the chain over the stay. Hold the rear wheel from turning and press on a pedal, and you;ll see that even when straight, a chain running on small sprocket isn't very high above the chainstay. This is why many mtn bikes have chainstays rerouted away from the chain, either bending down or run very high above on the way to the rear dropout.

Odds are that that the chain will slap the chainstay if you coast on bumpy roads or trails, so consider a chainstay protector to deal with it. These can be bought or improvised for very little cost.