Was the light on solid, or was it blinking? A P0305 (or any P03XX code for that matter)with a blinking light needs immediate attention.
If it was on solid (memory code), I wouldn't kill myself tracking it down now. That being said if I was worried about it, I would pull the coil and plug and inspect for any carbon tracks, ck coil connector for corrosion/loose contacts. Most of the missfires I ran into were dues to 1)plug wires, which you don't have and 2) plugs then 3)coils. Injectors rarely cause problems, not impossible but very rare. At the minimum if everything looks OK, I would swap my #5 coil with the #6 coil, or whichever coil is handy so if your ck eng light reappears and throws a P0306, then you know you have a bad coil. You could even swap the spark plug into another hole and watch for that cyl missfire code, me I would just grab an extra plug and put it in #5 so I wouldn't have to worry about it. Without a nice diag machine to do a running power balance, relative injector flow, and relative compression the "swap component method" will work, albeit slowly.
