Bear in mind Intel processors are arse-about-face (otherwise known as big-endian). Running on a little-endian processor, there is a clear "top" and "bottom". So we can define forwards and backwards.
But on Intel, let's say I want to write the number 1,234,567,890. And my processor has a 3-digit word size. It actually physically exists in the system as 890,567,234,1 ! So where's the top, bottom, front or back?
The other question, of course, is does the address register increment or decrement faster. There's no reason why those two operations should be equal cost (there's no reason why they shouldn't be, either :-) And if they're different, the result will be a difference in speed going forward or backwards.