i had allergies for years and never mentioned it...
....(but then i do'nt fly for a living but have a CPL and class 1) and one day they went away...so perhaps best not said (though strictly speaking i suppose you should conceal nothing)...
i suppose you have to look at ......a).... are you on medication for your allergic reactions. that would be important bacause some antihistamines could make you drowsy....some may not...( i am not a doctor but used several over the years)
furthermore...b) ....what kind of reactions do you get..??sneezing/running nose/streaming eyes....if the reaction was one that might affect your flying abilities...then it would be important to mention your allergic reaction....despite the possible consequences.....
maybe a medico on here would answer you to help to remove doubt. it might well be that the doc at the medical would consider your condition not to be a problem and might even suggest an antihistamine that the licencing authority would consider acceptable to a pilot for occasional use .....in which case you would feel better for not concealing anything.....
from my recollection, i was told that most people are allergic to house dust mites ....
always i think best err on the cautious side....