Yes, Apple suggest to use images with 22px, 44px and 66px size for UIBarButtonItems
, but if you use preinstalled icons like Bookmark icon, it's sized by 25px 50px and 75px for 1x, 2x and 3x respectively.
Here are 2 icons in UIToolbar
. On the right is Apple's system bookmark icon and on the left my custom icon.
Here my custom icon sized 22px-44px-66px:

And here 25px-50px-75px:

So, if you use custom and system icons in one toolbar, I would suggest to use 25px-50px-75px scaling, or your custom icons will be smaller. In fact I always use 25px-50px-75px scaling, it looks better on toolbars, as for me.