you aren't putting any "stress" on your modem. I ran an EMTA at 52db for years and it was fine
in fact, having levels at or near 50db can actually help improve your service because your the signal to noise ratio is increased for your return which is the most impacted by any noise in the plant.
that said, in Vegas it gets really hot in the summer and I have seen 3+db swings in return levels when things start to heat up and the AGCs in the amps struggle to keep up with the attenuation changes in the plant. This means what is 51db now could be closer to or above the 55db limit for the DOCSIS standard when it starts to warm up and at that point you will likely start to notice service impacts.
regarding your concern about correctable errors, I wouldn't worry too much. As long as the CMTS is able to correct for errors your service will be fine. What you want to watch out for is uncorrectable errors above around .1%...though you can't really see this from the modem, the Cox guys have tools to view this.
from what you describe, I wouldn't be too concerned. Just keep an eye on it as it gets warmer and if the return levels start to rise or if you start seeing lots of T3 or T4 errors each day or some other indication that your service is hinky like packet loss, then you are OK letting sleeping dogs lie