Hi! When you buy a Jonin Medal will depend on what you are looking to benefit from the medal. The Jonin Medal can do a lot of things and save you a lot of time and of course buying the Jonin Medal can give you rebate if you meet the required level for the event. However, one of the other things Jonin Medal can do is give you extra experience.

As someone who starts in a new server, experience to level up might be more important to you than waiting for that ingot spending rebate. The reason why is, the faster you level up, the faster things like events open up to you and the earlier you can take advantage of it. It is true, the rebate is better if you meet the requirement, but if one of your objective is to use the Jonin Medal to get an early advantage, then you may want to use it earlier =p

This is just my opinion and I believe it depends on what you want to do.

~Jib