Photoshop allows you to place a guide anywhere, which can be frustrating if you do Web work and you want to cut an image by hand.
Because your selections will miss the guides…
…and the things you create will have fuzzy edges.
But Photoshop will only select things by the pixel. So, the key to getting your guides right on the border between two pixels is to select first…
…then create your guides, snapping them to the edges of the selection.
Of course, you’ll need to make sure your Snap To is set.
Do it this way, and the things you create will have nice, clean edges.