It's funny that you mention MQTT, as the spec requires strings (including topic names) to be encoded in UTF-8: http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/cos02/mqtt-v3.1....

Granted, ASCII is a subset of UTF-8, so as long as you control all the publishers and enforce the ASCII-only rule, you should be ok. But if some day you need to integrate third-party systems and they use characters outside of ASCII...