If you've got extra pins on the backplane, putting some decoding outputs on them seems like a pretty good idea to me, especially since the signals are often already lying around anyway if you're using a '138 or similar. Use decoding that's not being used on the main board, of course (or at least for peripherals that can be disabled with a jumper) and then you can try out additional I/O chips without trouble.
I find it mildly annoying that on the SBC I'm mainly using these days (
RC6502 Apple I Replica SBC), where $8000-$CFFF is entirely unused, every 4K block in the upper half of RAM is decoded but most of those signals go nowhere. I want to map some of the alternate banks of the EPROM into that, but I'll need to add another '138 to the board that will do this, unless I want to start soldering jumper wires between the two boards, as well as using the bus connector. (I need a few anyway, though, so hmmm....)
This an idea used even in off-the-shelf, non-backplane systems. The Commodore 64, for example, had four different decodes going out on its expansion/cartridge port: /ROML ($8000-$9FFF), /ROMH ($A000-$BFFF or $E000-$FFFF, depending on the current memory map), /IO1 ($DE00-$DEFF) and /IO2 ($DF00-$DFFF).