I recon it is, but, I know you NEVER EVER can be sure of the clients environment. So have you ever encountered a browser that doesn't supports the arrow keys for scrolling the page both horisontally and vertically?
I also think selected form elements and flash objects disables the scroll, but is that always the case?
(This question is for a JS game, not an ordinary web page if someone should wonder)
The iPhone and Wii browsers spring to mind immediately. Other phones / set-top-boxes / platforms without a physical keyboard might well have the same issues.
(On the Wii, the "arrow keys" (d-pad) do scroll, but users are taught to drag-scroll using the Wiimote.)