In OS 9, each arrow needed a new click-and-hold. Holding the mouse down and dragging off the original button caused nothing to happen. So switching directions meant taking your finger off the mouse button and re-engaging the opposite scroll arrow.
All of the arrows (I prefer a double-arrow at each end) seem to act as one button while you hold the mouse down. However, the horizontal arrows are independent from the vertical ones. This is a very small thing, but it may be useful to you if you like to fine tune your view using the arrows!

