Originally Posted by swtsae
We had those size gaps because the person helping me who has done sheet rock before said "If you're tiling over it, it doesn't need to be perfect just a flat surface for the tile to sit on."
Were they wrong?
I wouldn't worry about it much. I've filled gaps larger than 1/4 before. I think what sdsester was saying is to be aware that when filling larger gaps, the joint compound will shrink when it dries. That just means that if this wall was not going to get tile, you would need to apply another coat of compound, to fill the shrunken space.
At any rate, your're tiling right back over it, so you don't need it to be pretty. As long as it's flat enough for the tile to lay smoothly, you'll be fine. If it were me, I would fill the joint with 45-minute setting-type compound, then lay mud and tape. Next day, I'd do my finishing coat of mud, dry, then apply tile.