The other part of the release mechanism is the cleat-gripping pedal hardware. Shimano pedals use plastic, carbon, and steel parts, and constant use can grind them down. The parts may also become loose and vibrate. If the mechanism loosens, the fit between the cleat and pedal is not secure enough and engagement and release suffer. Additionally, you may develop an annoying squeak that's most noticeable when climbing. The noise is caused by steel to steel contact of the cleat and the pedal backplate, rubbing against each other. You can temporarily quiet this by applying oil to the pedal and cleat, but if the noise is chronic, a more lasting solution is to replace the hardware. Oiling SPDs is considered regular maintenance, and the cleats shouldn't need replacing until you experience difficulty releasing them. Service while in use should also be considered derigueur; use a tire lever to clean chunks of mud out and a squirt from your water bottle to flush out the cleat.
SPDs don't allow for sideways shoe movement, but it's possible to create some by machining the rear jaws (the notched ones) on the pedal slightly. Use a small handheld grinder equipped with a small cylinder shaped grindstone bit to widen the notch. Avoid making the notch deeper, which can create slop. Work slowly and with care, and measure progress by testing the shoe on the pedal.
The pedal bearings should be repacked annually if your bike sees a lot of use. The grease seals that keep dirt and dust out can also keep water in. A monthly check for free pedal rotation isn't too often; slowly turn the pedal on the axle, feeling for any binding that might indicate a rough bearing surface.