You don't have to do it every level, but it can be done like that also, just its gonna waste a lot of events by doing that, imagine if you have a game with 200/500 levels ... it would be impractical... doable but impractical.
Made you a Array based 5 by 5 checker and reward, plus bonus% at level up.
All you need to change is change value of 5 from the global variable to denote your wanted reward level.
bonus reward is currently at 2 as (2*100)/100 = 2% if u want 1 % u just change the value of 2 in global variables.
If you want to adjust the count of rewards or how many times it should be rewarded you just decrease the array width or increase it, i made it for 1250 levels that is 250 bonus rewards given. or around 2500% health increase (if player reaches level 1250).
Have fun! ^_^