It all depends on the amount of slope involved. You can either have one level of split it into tiers. Whatever you decide the decking itself must be laid level!

We decked the bottom of a slope to take away the dipping/ditched area at the bottom of slope and about a meter of the slope - have a look at my pics - We really find the area useful - When we decked it we weren't sure what we were going to end up with but were really happy and 5 years on still are