1. Remember good ol' depth of field? Well, if you focus with the enlarger lens wide open and then stop down a couple of clicks, you've increased the depth of field and nicely corrected for any inaccuracy in focussing.
2. Stopping down increases the total required exposure time. This does two things:
A. Ensures that at low degrees of enlargement, the exposure time will not become unmanageably short, and
B. Enables you to have ample time for dodging, etc.
With time, you'll probably find that you're most comfortable with exposure times of 15 - 30 seconds and you'll stop down as required to achieve them.
Speaking of degrees of enlargement -- light obeys an inverse square law.
This means that if you record the correct exposure time and head height for a negative enlarged to, say, 4 x 5, you can easily calculate the exposure for any size enlargement. Just set up a way to determine the distance from the film plane in the enlarger to the easel surface and record the distance. I use an old yardstick, though a scale taped to the column will also work. Make up a set of tables with your basic enlargement size [8x10?] as 1.00 and then compute the multipliers for other film plane to easel surface distances.
By the by, f-stops on an enlarger work the same as those on a camera lens. If you change from f8 to f11, double the exposure time.