The opening of your speech has always been deemed über-important because that's when you have everyone's attention. Audiences also use those first few seconds to decide whether or not to even listen any further.
It turns out that your "audience" starts deciding all about you as you get up and start moving toward your starting point or podium. People create a first impression about you as their minds place you into the role of "speaker." Then, that impression impacts how they interpret everything you do thereafter.
It's not fair; it just is.
Not everyone agrees with this conclusion. Some claim that the first impression is not as strong as many believe. Others think that we get a chance to form a "second first impression" after a first meeting. Even if this is true, why take the chance?
Speakers and sales reps would be well-served to err on the side of a good first impression.
More importantly: your mom told you years ago it was the right thing to do. She's watching.