Should I use up Golden Apples?

It's currently half ap for ember farming right now so its good to burn them apples. However, if you have no one to raise then just save them.

or just save some ember and put them on 2nd inventory for future use

Its more worth to save for lottery events as they give mats in addition to the all xp cards. But honestly its up to you. Im using a few apples myself since i have more apples than i can possibly consume

Hypothetically, if you saved your apples for a roulette you would benefit more but that is just not realistic most for most.

If you plan to open around 100 boxes or something huge and will make plans to ensure you have the time and energy to do so than it is not a bad idea but I suggest using them when you need it.

If your need for XP is immediate, then I would say it's worth burning a few but not all of them. Otherwise save them, as you can get more value out of them during a lottery event.

Its up to you to use your apples if you have a lot of them from the previous year but remember.. theres a Nerofest (definitely not) Rerun on june (i think).... so stack some apples as you can then go rampage at the event cuz its definitely WORTH IT!