Do I need a chain guide or bash guard to convert to a single chain ring? I want to convert an old 12-speed into a 1x6 for urban commuting. I'm hoping to keep one of the existing chain rings at 42T and use washers on the chainring bolts or replace them with single speed bolts. When I remove the front derailleur, do I need to replace it with a chain guide? Do I also need a bashguard? Or is that more for downhill mtb conversions?
I agree with Batman.  You can disconnect your front shifter, and use the indexing screws to position the front derailleur to act as a chain guide, and you're good to go.

If you're not interested in doing that, I would recommend paying attention to Emyr's advice--chainrings intended to be used with a shifter have "ramps" on the gear teeth to facilitate shifting.  If you're not using a derailleur, these can make your chain drop, which is bad.  You might want to get a singlespeed-specific front chainring, they're not expensive.  

But, if you keep your derailleur as a chain guide, you'll be fine.