As far as what is considered low, IIRC, TPMS will indicate low pressure when they are ~5 psi under. Most will also state when they are too far over as well. 31-32 psi is a bit lower than the recommended 33 (obviously), but it's nothing which is going to kill your tires. If I noted mine being there, I'd put it on my list of things to fix and get them at or above the recommended levels. There is nothing which states you must have it fixed when it gets xxx psi below recommended. Just the general guidelines of if it's below what's on the sticker, put some air in it (at least I've never heard of a tried/true number ... there may be recommendations out there, but I believe they are going to be opinions and nothing quite set in stone).
I usually ride my tires a little over the stated amount by 2-3 psi. This ensures I'm always at or above the recommended amount. Beyond that, it gives me two benefits I can state. First, there is a little less rolling resistance, which means I should be getting a little bit better fuel mileage. Second, they give a little less sidewall deflection, which adds to stability.
Something to consider with higher than normal tire pressure, it also decreases the contact patch, which means it decreases traction. This may not be the best situation during the winter months if you drive in snowy conditions or on snow packed roads.