When it is clear, maybe - but most of the time the trap that questions fall into is seeking legal advice that we can't give.

Most of the time the only answer without possibly giving them harmful or misleading advice is "Ask Canonical" - http://www.ubuntu.com/legal/terms-and-policies/contact-us - that address specifically for how Ubuntu and it's logo can be used.

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Examples (against):

http://askubuntu.com/q/666635/47291

http://askubuntu.com/questions/164616/can-i-use-the-ubuntu-logo-on-t-shirts-produced-non-comercially (quoted text has changed, many say things like "It's Fine!" - when it may not really be...)

http://askubuntu.com/questions/361889/am-i-allowed-to-display-the-ubuntu-logo?rq=1

http://askubuntu.com/questions/106735/is-it-possible-to-create-ubuntu-shirts-merchandise

http://askubuntu.com/questions/592514/can-the-ubuntu-logo-be-used-for-my-business?rq=1

http://askubuntu.com/questions/592442/is-it-legal-to-use-ubuntu-name-for-my-cybercaf%C3%A9?lq=1 - three link answers saying "ask"

Examples (for):

http://askubuntu.com/questions/58428/can-the-ubuntu-logo-be-used-for-a-commercial-website - answer is clear