Ya, I have had to ballast my cirrus when I had a very heavy passenger up front with me to keep the plane from being too heavy in the front. The problem when a plane is out of balance if you get into a stall, it could be unrecoverable. I've landed the aircraft with heavy front COG and was really surprised how quickly the nose came down on the runway, compared to the normal flare and float I was used to. : )

While training you need to know how to calculate W&B by hand on paper, but there are various wonderful applications on the iPad or iPhone that do it for me quickly. Here is a simple example of one if you're curious.

https://ipadpilotnews.com/2014/10/calculate-weight-balance-f...

The problem is when having not enough load in certain areas of the plane its center of gravity gets out of limits. For example it is quite common for small sailplane pilots to load some kilos of lead in designated parts below their feet to balance the plane.