Do I need short grain paper for bookbinding?

No.

There, I’ve given you permission* to use any paper you have at the ready. 

If not having short-grain paper is preventing you from starting, get over it! The key is to start.

I’m afraid my book will burst into flames and disintegrate before my very eyes if I use long grain paper!— Won’t my book explode into shrapnel because the grain direction is East-to-West instead of North-to-South?

It will do no such thing. In fact, you will be hard-pressed (pressed, get it?!) to tell grain direction after everything is assembled and your book will not fall apart upon one, two, or twenty read throughs. Two things that will happen: (1) the swell (thickness at the spine vs. thickness at the edge) will be slightly more, no big deal (2) the crease at the folds will be slightly less clean, but the crease is way down in there and will barely be seen.

But I’m a perfectionist, and I always do everything to a very high standard! 

In that case you are going to make a perfect test prototype. Yes, the book you are making is not the REAL book. No, that one will be perfect. This one is just the prototype and, because you PLANNED on it not to be perfect, it is okay however it turns out. Now, you are merely perfecting your sewing technique, understanding the mechanics of casing in, and preparing for main event later. It would be wasteful to use short grain paper on the prototype. Go ahead and use whatever you have on hand.

I don’t like the paper I have.

That is a very different matter. If you don’t like the paper you have to work with, if it does not “spark joy,” get new paper. You should enjoy yourself during the process and if your materials are working against you, that’s not ideal. HOWEVER, the paper you get still doesn’t haven’t to be short grain. Maybe you just don’t like the texture or weight of your printer paper. That doesn’t mean you need to order a box of Mohawk Superfine. Consider a ream of long-grain paper that feels more mmmmm on your fingers.

It’s for a gift. I feel bad about cheaping out. 

See above. I bet that if the paper feels nice to the fingers, then your recipient will probably have no idea what direction the grain goes and will instead cherish the thoughtfulness and the work of your heart and hands. If you still feel guilty (“but I love them to the end of the Earth, how can I do anything but the maximum to prove my worthiness of their love?!”)… splurge.

So there you have it. Long Grain Paper is a perfectly good option. You have my permission* to use it! 

*If you were NOT looking for reassurance that your book wouldn’t melt into unreadable goo should you heretically use long grain paper and were instead looking for someone to give you an excuse to splurge on some buttery Mohawk short grain… then you definitely need it and you should definitely get some.