The first to ask is never a waste of time because there are people out there who don't understand what the legal jargon means.It's polite to say thank you, but I would say that asking for permission when permission has already been explicitly granted is actually a bit rude. It's wasting someone's time, implying that they have an obligation to reply that they have taken steps to avoid. If someone posted on their web site and official forum thread "You can use this in a mod pack", would you still ask them if you could use it in a mod pack? Same thing.
Asking once, in a public location can provide an easy place for people to get a quick answer.
Now, if people continue to ask after it's been explicitly asked and answered, then that's a waste of time.