Title says it all. See this answer. I have bolded the questionable part of the quote.
Burn Ubuntu to a DVD and you won't have any trouble booting it... glad that worked for you. Please rate and accept my answer.
No, that is not acceptable. What I would find perfectly fine and acceptable is a comment asking for acceptance if the OP said the solution works but they didn't bother to accept any answer.
That is as much noise as "Please help" in a question, with the added "fault" that it might even imply begging for rep.
Edit: besides, that's not even an answer. The OP asks how to fix USB boot errors and the answer reads "burn a DVD instead".
In my opinion, it's not mainly about what you say but how you say it.
While reminding new users who might not be familiar with the site yet to accept an answer if it solved their problem, this should always be worded as friendly suggestion, never in imperative and never unconditional.
Accepting an answer or not is still only the choice of the question author, and nobody is allowed to force them.
However, the answer is not the right place to put this anyway, as it does not help with the problem described in the question. It would be better suited as comment below the answer.
I personally leave a comment more or less like this below my answer if it is the only (valuable) answer to the question of an obviously new and inexperienced user:
If this answer solves your problem, please consider accepting it by clicking the grey round check button on the left of it. Otherwise feel free to leave a comment if something is still unclear or not working as expected. You should also invest two minutes to take a [tour] of our site and learn how it works. Thanks and welcome to Ask Ubuntu!
If there are several valuable answers though, and I am polite enough not to highlight and advertise my own answer, but leave a comment directly below the question instead so that the OP is not influenced in their choice which answer to accept:
If any of the answers below solves your problem, please consider accepting the one you like best by clicking the grey round check button on the left of it. You should also invest two minutes to take a [tour] of our site and learn how it works. Thanks and welcome to Ask Ubuntu!
I've nudged OP's via comment to take the question off the "unanswered list" by accepting an answer, stating what is not working with an answer (so it can be amended with improvements) or posting their own answer. Who's answer is accepted is a moot point because the important thing is to take the question off the unanswered list.
Of course our egos all want our answer to be the accepted one but the right thing is for the right answer to be accepted. Occasionally based on OP comments you have to badger them to write their own answer.