For cards with optional effects (using 'may'), are targets in those optional effects still mandatory to declare when playing the card?

Ruling: Yes, targets in optional effects using "may" must still be chosen when finalizing the spell. If there are no legal targets for any part of the effect that involves targeted game objects, you cannot play the spell at all.

Nuances:

• The "may" decision is made on resolution, but target selection happens when finalizing the spell
• This applies even though the effect itself is optional
• A target on the same battlefield as another target is a legal target choice
• Cards using "up to" work differently than "may" for this purpose