Auto bed leveling questions

I just got mine up and running using this style [www.thingiverse.com] . Its a great idea but the springy bed causes to many accuracy problems for this to work properly IMO. I plan to switch to a sevo mounted sensor style. I am using Marlin, Slic3r and Pronterface with a Ramps 1.4.

Anyway i have a couple of questions that i cant seem to track down the answers to.

1. Does Marlin simply take all the probe positions and try to figure out the best solution for a flat plane? In other words is the "grid" that it uses a flat plane grid or does the grid go up and down in a straight line depending on the probe points? If its simply a flat plane grid then i cant see any point to probing more than 3 points.

2. Is there a way to probe the bed once and store those settings into the firmware or do you have to probe the bed before every print?

The probe command is part of the Slicer custom code. It's the G29 command and has to be in the beginning of every print.
I dont use more than 3 probes. I assume that the multiple probes take every measurement into account so if you probe 20 times then the machine will build a complex grid of heights going up and down. The 3 point probe only calculates a plane. There would be no point using the multi point probe if it only calculated a single plane.
Regards
Mike