When to Treat Prehypertension

I would be interested in hearing people's opinions on this since I have a doctor's appointment soon.  I've had blood pressure in the prehypertension range that was discovered maybe 8 years ago now.  I am only 26, a healthy weight (128lbs - female), don't smoke, and eat low salt diet.  I also get moderate exercise as I live in a city and walk home pretty much daily (2 mile walk, ~ 40 minutes).  I'm pretty sure this is all genetic because both of my parents are on hypertension medication, but not until their late 40s/early 50s.

Anyway, I tend to get sky high readings at the doctor's because I freak out, so I bought a digital home monitor.   my readings usually range from about 115-135ish/78-88ish.  When I first sit down (without resting for a few minutes) to take my pressure the first reading is also often quite high, in the mid-upper 130s and often low 90s.  This makes me very worried that my "normal" blood pressure during the day is probably a bit high.  Should I take comfort that when I'm sitting quietly for a few minutes I can hit pretty good blood pressure readings, or should I worry about the possibility of it being high  when I'm not taking it?  

I'd hate to start taking medication now in my 20s, but I also don't want to damage my body.  I need to figure out if it's something I should pursue further with my doctor or let it go because she hasn't seemed too concerned yet over it.  Opinions please.