Which one should I do low or high?

tell me about your cluster - does it have alphanumeric display, that says "fasten seat belts" or "washer fluid low", or you have a pictogram of a car, that just shows a picture of a car, with trunk lid open, or door ajar? if you have alphanumeric - it is high, otherwise - low... also, low has only one reset button for tripometer, high has two buttons, one for tripometer, the other for computer messages...

I have the alphanumeric, I'll check it tomorrow once I get a chance to get it in the garage

The way parts store machine checks a battery is to charge it and then perform a load test. It is not necessary to do a trickle charge/ deep charge to check the battery. You can check the alternator at the battery cables by reading volts at the battery while vehicle is not running and then running. Should read 13-14 amps when running.

I just checked the battery voltage from my cluster and it read 13.5-14V while idling.

Is this right or should I be worried?

Nothing wrong there.