First, never volunteer any information, always be the silent type and do what your told by the staff at MEPS.
Secondly, if you have any type of health record on anything and you dont report it, if that particular injury/condition becomes an issue during your service, you'll find yourself in front of a court marshall defending yourself as to why you did not list that on your entry papers because the military WILL search your health records all the way back to your birth. Now dont report sprained ankles and similar ills, but do report broken bones, stitches, etc. I stated that I had a "finger in the eye" during football and I had to produce those papers even though it was a VERY minor injury. Be honest bout your allergies, I doubt they will DQ you because of a small case of hay fever. If there is no record of it in your health file at the hospital/clinic, then I say dont worry about. Just dont stash any allergy pills in your kit at boot camp otherwise you'll be dead meat.
Thirdly, when the dock say to "bend and spread cheeks", dont lean forward and grab your face cheeks to show him your teeth and gums, that will piss him off and give him even more of a reason to DQ.
Forth, drink gallons of water the day befor you go to help your system "purify". It will help lower your blood pressure and clean you piss of any extra salt, glucose (sugar), and toxins that might raise eyebrows that question your health. They can read alot about your body by the piss sample. Dont wink at the guy when you hand him your sample, either.
Fifth, just relax and soak in the oath that you take. Most dip-f*cks that take that oath have no clue what they are saying or doing.
I was DQ'd due to a "questionable" hip condition that I had when I was 5yo, otherwise I would be jumping out of airplanes with the 82nd AB. Oh yeah, I volunteered that to be "honest". I played college footabll, lifted tons of weights, and ran miles and NEVER had any pain or complications because of my hip. The Amry missed out on one of the most dedicated soldiers they ever could have had; Me. I still get pissed when I think of it 9 years after I went through MEPS.
Good Luck!!!