transtemporal
Explorer
For me, have to take the extra step of asking the player whether he wants to use a luck point every time he rolls or I roll against him adds a completely unnecessary step that over the the course of an entire session or campain adds up to a lot of wasted time. Besides I've never noticed any imbalance in letting them roll after the the result has been announced.
All you have to do is pause slightly after the player announces their result but before you announce whether its a success or not, or the reverse if its your roll. Think of it like bluffing in Poker. The player may interpret your response that they missed and spend the luck point, or they might not. Allowing them to decide after you announce the result makes the ability too powerful. Thats not "Lucky" thats "Oracular". Or something.
I would only hint if its a life and death roll (like a mega-crit), as in "The Orc howls in triumph as it swings its greataxe towards your neck... <pause, meaningful look>"