What should the voltage of a fully charged lead acid battery be?

See my stack exchange answer to "Lead Acid Battery Charger Design Factors" which relates, and follow the link there to the Battery University site which will tell you far more than you knew there was to know about lead acid (and other) batteries.

From the above answer note the quotes from the above website. Especially in this context

• The correct setting of the charge voltage is critical and
  ranges from 2.30 to 2.45V per cell.
  Setting the voltage threshold is a compromise, and battery experts refer to this as “dancing on the head of a needle.” On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate; on the other hand, an over-saturated condition causes grid corrosion on the positive plate and induces gassing.

  To make “dancing on the head of a needle” more difficult, the battery voltage shifts with temperature. Warmer surroundings require slightly lower voltage thresholds and a cold ambient prefers a higher level.

2.30 x 6 = 13.8V

2.45 x 6 = 14.7V

13.8V is the nominal voltage that many automobile systems operate at.
14.7V is higher than you'd usually meet but around 14.4V is common for "topping" mode charging which is used to equalise charge in series connected cells. See the Battery University site for MUCH more information.

If you charge to only 12.6V as several people have recommended your battery will not ever be at full capacity and will have a shortened life.

See also Safe operating area for different types of battery chemistry?

And also Can I charge a 12v sealed lead acid with an old wall-wart (not made for charging)?

And also Charging lead-acid batteries?