I disagree with prepaying taxes, insurance, HOA fees, etc. Once you push forward, you have to do it in perpetuity, but you don't get the boost in subsequent years. If your mortgages are at fixed rates, then your rents will continue to rise, but your payments won't, so the problem gets worse over time, not better. Not really useful in the long run. And postage? You should be doing everything electronically.
Maintenance is another story. Staying ahead of appliances so they don't break, or continually freshening up the property so the house is easy to rent is not a terrible strategy, but if you're in it for the long term, even this isn't all that helpful.
Luckily for us, random things just seem to break. A motorized awning on one of our houses just quit and cost $1400 to repair! A new refrigerator in another house broke while under warranty, so we had it repaired. Then it broke again, just after the warranty expired. The MFR will supply the parts to fix it again, but wants $350 for labor. WTF?
IMO, you're better off having a large cash cushion, so that when shit breaks, you don't lose sleep over it.