I will be only doing the fronts because i didnt remove the rear brake lines. So even before starting it i remove the cap and fill it to the recommended line. That should be enough, why would it ever run out of fluid and then suck all the air in. Thats alot of brake fluid. Once you open bleeder isnt the whole process like 10 sec at most not even since pedal goes to floor quick.
You can very easily run the master cylinder dry and start sucking air. It would only take 8-10 strokes of the pedal to drain it.
To properly bleed the brakes, you should do the rear. Bleeding the brakes is an important part of maintainence that nobody ever does. You should do it MINIMUM ever 2 years. I do mine yearly and each time i notice a huge improvement in pedal feel.
Reason you do the rears first is to purge the old fluid from the longest lines first and then work to the shortest lines. It will take a number of pumps to do this, so it's why you need to have someone monitor the MC and keep it topped off.
Even if you only want to do the fronts, you should do it enough to get the air out and flush all the old fluid out. The more old stuff you get out the better.