Should I Winterize if I use it during winter?

I plan on using my trailer all year round. If I empty out the water from all sources, will I be ok or do I have to put the RV antifreeze in? If I have to use antifreeze, do I have to use it every time I come back from a trip?

It's so easy and takes so little time, you'd be risking a bunch not to do it each time.  Also, keep in mind, you  will get to a point in the winter season where you will not be able to properly/safely use all the water related items while camping.  What I mean is, if it's only dipping below freezing for an hour or so at night then getting above freezing during the day, you should be fine all around.  Once it starts to stay below freezing longer and does not break freezing temperatures during the day, you'll need to stay winterized with antifreeze throughout all  lines.  You won't have running water inside or will you have use of the hot water heater.  You will be able to use the toilet by flushing by dumping a pale of water down to flush, but make sure you have a couple gallons of antifreeze dumped in your holding tank.

Also remember that if you use Pod water, not only do your fresh water pipes and pump have to be re-winterized, but also the waster water tanks hang from the bottom of the pod and will freeze up.
When winter camping...I use a porta-potty and/or the campground rest rooms. The porta-potty sits in the foot well area of the bathroom.  I carry a couple of five gallon fresh water jerry cans in the Pod.  I heat water on the stove and wash dishes in a rubber tub that fits in the sink.  I can then carry the wash water out to dump it.