Is is safe to let my PWM charge my 12V battery at 14.8V?
Check the battery datasheet for proper voltages that are exactly correct for your battery. Otherwise use safe values. 

I have a commercial solar battery charger that by default overcharges the battery once per month to 15 volts to equalize the cells. Otherwise it charges it to 14 volts. These voltages can be changed from the user interface.

If you use too high battery voltage, it will cause gas formation in the cells. Traditional wet batteries will dry, gel batteries not so much as they contain chemistry to minimize it. But they will suffer and the end result is diminished battery life.

The most important thing for me has been to learn that it's not a good idea to overcharge the batteries. While it "feels" like you have more charge in your batteries when they say 15.0 volts than 13.5 volts, the difference is quite small in reality, as the last couple of percents of charge raise the voltage very quickly compared to the normal operating conditions.