Surprise bp check at dentist. was incredibly anxious before appt and more so during the surprise check. took as soon as i sat down. 128/74. had two mugs coffee earlier. thin and healthy, brisk walking 7 miles per day. can i ignore this high reading?

I would not consider this a high reading in the setting as described. Feel free to invest in a home BP Monitor and take your BP several times/day and several days in a row, preferably when you are relaxed. This would be a more accurate reflection of what you have. Stress brings out stress hormones that raise the heart rate and pressure. In this setting it is called white coat hypertension.

In the described circumstances your blood pressure was lower than it could have been. If you have a sense of where your blood pressure is in normal conditions, and that value is normal, you can definitely ignore this isolated reading.

Given the circumstances you described, your blood pressure is not meaningfully elevated. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs.