I have a question whether if one begins to use a watercolor block for drawings, if the paint stains the sides of the other sheets in the block by the edge, or does the glue prevent this? I wondering b/c I don't have much space for taping down paper, if it's even worth the extra cost and less clutter.
The glue might prevent this (depending on if the glue really is spread perfectly along all the edges), but a visual inspection should be enough. More likely is that the surface tension of the water will keep it from spilling over the edge.
If you are really worried about this and your technique requires huge amounts of water on the surface of the paper, you could apply some removable liquid frisket to the edges (available online or at your art supply store [e.g. windsor&newton]). Test it to make sure that the brand you are using does not damage the surface of the paper when you remove it.
Commercially made watercolor blocks do not stain the sheets below the one you're working on, nor do they stain the edges of the paper (except for the little 'not-glued' bit. It's possible (though it's never happened to me) that you could get some staining at the little open bit that's left in order to enable you to remove the finished painting. It would be minimal if at all.
Watercolor blocks are EXPENSIVE. I love them, so I make my own from watercolor pads. You can purchase "padding compound" online. I get mine from Dick Blick, but I'm sure other places--Cheap Joes, etc.--also carry it, or you could look at Amazon. Once you buy a bottle, you'll probably never need another one--it doesn't take a lot. Search Youtube for tutorials on turning a pad into a block. It's easy. That said, if you're not careful, you can get some glue into the edges of your home-made block. That's never been an issue for me when it's happened. The quarter inch or so at the edge of your painting is going to be hidden behind the mat if you frame it.
I don't personally like any size bigger than 10 X 14 in (26 x 36 cm) for watercolor blocks. They tend to ripple too much and may even separate in places from the glued edges.
Second suggestion: You can tape down the pages of a watercolor pad TO the pad--whether spiral bound or glued. I use painters tape--white or gray if you can find it (to keep the color from confusing your color sense while painting). If you're careful to keep the width consistent, you can create an attractive white edge around the paper. (Be sure to burnish the tape down with some hard smooth object like a plastic spoon to keep paint from bleeding under it). I put a piece across the bound edge too, to keep that edge white and complete the "frame."