The autopilot is not aggressive enough during leveloffs and you can lose separation if it's choppy. Realistically though, it's not like the 100/200 is going to be climbing so fast that a level off will shoot through your altitude. Maybe in the 700/900, I'm not sure. You only have 150' to play with on either side of your assigned altitude in RVSM.

Look under Ops manual page 5-57. It isn't prohibited, but it says "Extreme care must be exercised if using this mode."