No, the sum of 5.7 cm and 6.2 cm doesn't need to be rounded.
5.7 cm + 6.2 cm = 11.9 cm
Now,
Both 5.7 cm and 6.2 cm has 1 decimal place. So, their sum also has to be equal to 1 decimal place tooo. As you can see their sum also has 1 decimal place. So, you don't have to round it.