I think Main Quest has a lot of bond points so maybe you can use it there. Unfortunately, we don't really know what's the next event at JP (maybe another December event, but still) and we don't know if Muramasa has bonus bond or not, so if you want to continue main quest, then maybe you can do that.

I'm not too sure because sometimes, the Bond EXP is different and you won't be very free to choose the Servants you're using. (Forced supports, and the need to use Merlin-tier Servants to win against the difficult Main Quests.)

The best method of using them are either during the simple Interlude Quests (not the hard ones) or one of those level 90++ Free Quests you'd find in events lately.

(Strengthening Quests do not offer as much Bond EXP).

I'm not sure if they'd hold out till there's a Christmas event (will there be one???) or something though.

You can use it by clearing Tunguska (if you haven't), Traum, or the Road to Lostbelt 7 itself (Interlude and the Challenge Quest). But, if you already finish all of it you can use it on any Interlude the servants you haven't cleared (especially for lvl 60+ node). I do remember if Jeanne Alter Interlude can give you around 2k bonds, it's worth to use the teapot in there. Make sure you use all of it before the time run out, don't be like me whose regret didn't use it when Road to Lostbelt 6 began, cuz I think I could save it for later.. yet, I lost 30 teapot :(

You shouldn't rely on them though.

It's usually best to use them up on random Quests on the Servants you want ASAP if it becomes obvious that you won't be having the opportunity to use them on those special Quests which rewards high Bond EXP.

I made that mistake when they expired on me half way through the Halloween event. (ಥ﹏ಥ)

Main quest stages offer some of the highest bond gains in the game, way more than event nodes.

Teapots expire in a few weeks.

Muramasa has a currently has bond bonus for LB6 free quests.

Do what you will with this info…