No, use of character abilities is only optional if the text says "may"
If it tells you to do something, instead of saying you MAY do it, then you MUST do it.
This is most important in the case of Shelob. She returns to Gondor whether you want her to or not. "Instant kill" abilities are also not optional. Under very rare circumstances, it would be ideal to play cards instead of automatically winning, but your just out of luck :-)
Agreed... sometimes what I'm thinking in my head with all the extra parts doesn't come out entirely when I write it.
The "may" actions are choices, all other abilities are explicit and most are fairly specific. I think the full rulebook does a good job at the end where it goes through each character card in detail.