You would be much better off to put it in D and forget about it except in situations where you are descending a steep grade and you need engine braking effect that forcing it into a lower gear would provide.

If you don't use a higher throttle opening/engine load occasionally you will end up with a condition that affects all 4 valve per cylinder engines with hydraulic tappets--carbon build up that can cause a start then stall then won't start until you crank engine for 3 minutes at part open throttle. In my years of working on many makes of automobiles I have seen almost no troubles caused by driving a car moderately and using wide open throttle when it is needed and untold problems on cars driven on short trips/slowly/never at high throttle opening. Poking around and driving like there is an egg between the aceel pedal and your foot will not get engine to as warm a temp on cold days and the moisture that forms in engine(condensation) will not go away readily. This will sludge up the oiling system in time and will shorten engine life.

I'm not saying to go break speed laws and race everyone from a light but by all means DON'T BE SCARED TO USE THE POWER OF THE ENGINE AND TO ALLOW THE TRANSMISSION TO SHIFT UNDER HIGH THROTTLE.

There is a service bulletin on late model Volvos to address misfires and the aforementioned start/no-go condition that has the technician drive car in lower gear at engine speeds over 5000 rpm for several minutes to clean the exhaust valves of deposits.

Taking it in and out of gear while stopped is going to lead to more wear on trans as every time you put it in gear there is a brief period of slippage of friction plates in transmission. If you leave it in gear there is no wear item involved. Non-contacting parts of the torque converter are moving while you sit with car in gear. If you tow a trailer and drive in mountains and you were to see a scenic pull out immediately after a steep climb then leaving engine running with transmission in Park would cool fluid off. The fluid in trans gets heated by the action of the torque converter (except when it is locked on level ground/low load situations) but this heating is less detrimental in normal traffic situations than putting the trans in and out gear excessively. The wear on engine mounts and driveshaft joints is increased by unwarranted disengagement/engagement of gears in auto trans.

Drive your car without fear of hurting it with high throttle use.