Depends on what you mean by "safely." Deep cycle, lead-acid batteries will last longer if they are not routinely discharged too deeply. Generally speaking, the lower the average depth
of discharge, the longer the batteries will last. So routinely discharging them more deeply may somewhat shorten their life.
On the other hand, on a dollars-per-watt basis, you may get more out of the batteries over their lifetime by discharging them somewhat more deeply than you have been. This is because it takes less time (and therefore usually costs less) to recharge from 50% to 85% or so, than it does to recharge from 85% to 100%. Figuring all those costs out, over the lifetime of the batteries, can be a fairly complicated calculation.
In any case, there is no immediate danger
to discharging them to 12 volts. It's not like you are going to kill the batteries straight away by doing that. Lots of people routinely discharge their batteries to 12 volts (or even less) before recharging them, and still get many years of use out of them.