newb question on growing multiple heirlooms

[epsilon]

I was wondering. How does one go about labeling seed from heirlooms that are grown within proximity to each other?

If I grow five varieties and save seed from this current season how would I go about labeling, resowing, and evaluating the seed in the next season?

Say if this season I were to grow KBX and Black Cherokee in the same area, would I assume that the next generation of seeds that I plant would have picked up some amount of the DNA from a parent that was grown close or do I only make that call if the subsequent fruit showed some sort of deviation from the documented norm?

I ask this as I was told that I need to grow a whole bunch of different varieties of heirlooms this year, as opposed to my normal here's a beef steak and here's a paste.

Or as a normal small grower do I just make notes as what possibilities to expect from future F gens?

Or do I save seed from each plant and call it as such?

Or do I not worry because this isn't really my "game" yet and I'm really just in it for the tasty rewards of planting?

Gaston