The Night Hag’s Heartstone is described as:
All night hags carry a periapt known as a heartstone, which instantly cures any disease contracted by the holder. In addition, a heartstone provides a +2 resistance bonus on all saving throws (this bonus is included in the statistics block). A night hag that loses this charm can no longer use etherealness until it can manufacture another (which takes one month). Creatures other than the hag can benefit from the heartstone’s powers, but the periapt shatters after ten uses (any disease cured or saving throw affected counts as a use) and it does not bestow etherealness to a bearer that is not a night hag. If sold, an intact heartstone brings 1,800 gp.
We notice that the description says “can” benefit. This seems to suggest the user has a choice. The further addition of the word “uses” also seems to support that the user “can” “use” the object.
When carrying the Heartstone - does it force every saving throw to be at +2 and burn a charge or is it optional?
If it is optional, can the bonus be applied after the roll or does it have to be applied before the roll?
Note: It appears that our DM used the 3.5e version instead of the 5e description so that helps clarify part of the confusion. We have revised this to cover the 3.5e to make it a viable question that may serve others.