If I attack with Sceptile's "Power Poison" attack, do I have to discard an Energy card if I don't want to Poison the opponent's Pokemon?
It is not optional. Even if the damage would be enough to Knock Out the Pokemon, you still have to discard an Energy and Poison it.
Source: Primal Clash FAQ (2015-02-05), TPCi Rules Team (2015-02-05)

Relevant cards

Note: All applicable cards may not be listed below.
XY5_9