Depending on your engine/framework, this may not be necessary to have them in a sheet, trimmed or in a grid.
For example, Unity Editor does a good job of receiving individual untrimmed frames as the raw assets, doing tight hull packing on them, and using them as frame-by-frame animations with the alignment in tact.
This means you don't need to repack them every time you export, or need to deal with the additional Spine/libGDX atlas format to load them back out.
But as erikari pointed out, if you want smooth animations and efficient assets, an actual skeleton data export is usually the way to go. But for that, you need to use the Spine runtimes.