Summertime hot water in the cold lines
It is totally possible for thermal expansion to back some hot water into your regular cold water supply. A check valve will prevent this but you are correct in that this could cause an unwanted (potentially damaging) increase in pressure on the hot water side of the system.

You should look into installing a thermal expansion tank if this is enough of a problem for you as to consider installation of a check valve. It is a fairly simple and inexpensive solution; do an internet search and you will find a lot of information. Check with your local permitting authority to see if this is a DIY install, or call a licensed plumber.