Assumptions are always dangerous, in life, in business and in presentations. It is far better to find out the truth before hand than base your presentation on assumptions.
In technical presentations, in particular, do not assume that everyone in your audience will understand the technical terms, jargons and TLAs (Three Letter Abbreviation) that you may be tempted to use.
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Graham Young
A to Z of Business Presentations 
While assumptions are accepted practices in scientific studies involving some mathematical solutions, it is indeed discouraged in human interaction. I always share this anecdote when the topic infringes on the question “Why we should not ASSUME?” Simply put (pardon the use of slang).. we should not ASSUME because when we do we risk making an “ASS” out of “U” and “ME”. do this with the word written on the board for graphic impact.:-)
Just came across a list of the top 20 assumptions which proved to be wrong here: http://www.associatedcontent.com/article/5553346/20_of_the_worst_assumptions_made_by.html?cat=55
Thanks to Paul Sloane