No, the OP isn't required to use fill flash.
But given what we see, we think he might really like or benefit from it, so we are encouraging him to
try fill flash.
It looks to us like it may really suit what he does, and may add flexibility to his efforts.
If you are going to try it:
1) use X synch. Your C220 will synch with the flash at all shutter speeds, which is important when you start adjusting things to balance fill and ambient light;
2) use a flash with a built in "automatic" sensor. Over here they were often referred to as thyristor flashes, but that is actually a misnomer;
3) put the flash on the cold shoe on the grip;
4) generally, I would set my "automatic" flash to under-expose by one stop to achieve the right level of fill. That means set the exposure on the camera to match the ambient light, and then set the flash to give one less stop of light than would be optimum for the ambient light setting. Usually you would set things up with your subject being at least partially backlit.
In my case, I did or do most of this sort of work with one of the following:
1) initially, a Vivitar 283 flash;
2) mostly, with a large, really powerful Metz 60CT series flash - great for weddings;
3) more recently, with either the Metz flash or an Olympus T-32 or T-20 flash.
The Olympus flashes are slightly limited, because they aren't particularly powerful, but they are also relatively small.
This was done with the C330 and a larger, studio flash supplying the fill: