Rule 605.3a says:
A player may activate an activated mana ability whenever he or she has priority, whenever he or she is casting a spell or activating an ability that requires a mana payment, or whenever a rule or effect asks for a mana payment, even if it's in the middle of casting or resolving a spell or activating or resolving an ability.
Do I understand correctly that I can tap Pheres-Band Tromper enchanted with Karametra's Favor for mana in my opponent's end step even if I do not need any of it? If it works I could trigger inspired every turn without the need for attacking. Some years ago, mana burn definitely would have prevented this kind of play.