It depends on your tolerance for noise and choice of CPU cooler.
For the longest time and up until last month, I built all my systems with a side-panel blow-hole. You WILL get colder air going through the CPU heatsink fins, and you will also get more noise. Depending on your fan choice, whether you use noise-absorbent ducting or "foam-board ducting mod", and any number of things, this noise may include motor-whine or exclusively just air-turbulence. Because I wanted a high-througput CPU fan (more air-turbulence noise at minimum, even if no motor-whine), I closed off my duct, relying exclusively on the intake air from two slower-spinning, quiet 120mm front case fans.
With the ducting mod, I wanted the air-pressure from these fans to feed the CPU fan. I decided I could live without air from the side-panel, where the pressure is limited between the filter covering a single 120mm hole and no help from the front case fans because the duct separates the intake from the side-panel with the intake from the front case fans.
It took me a while through trial and error to get to this "optimum" combination for my recycled 90's vintage full-tower case. Your case has an advantage for the thoughtful placement of the rear exhaust 120mm fan port. If you can enhance intake CFM's from the front of the case at a very low noise level, you might consider blocking the side-panel port with plexiglass or PaxMate. But the side-panel blow-hole is one aspect of Intel's published thermally advantage case design -- a guideline to builders.
To you, it may factor into decisions about what CPU cooler to use, and what sort of fan should be on that cooler. I think if you were to use a larger CPU fan like a 120mm with an XP120 cooler or a cooler with a fan-adapter (higher-throughput, lower noise) you might be inclined to keep the blowhole, even if it is narrower than 120mm. Actually, that discrepancy could enable you to mix side-panel intake air with front-panel intake air. At one time I had a 92mm duct feeding a 120mm CPU fan, which was drawing air from around the duct as well as through it.
If you only have 80mm front intake fans or even one or two 92mm fans, you may be more inclined to leave the duct open.
My experience with putting fans on the side-panel, or even securing them to an internal case frame so that they sit near a hole in the side-panel, is that -- well -- I didn't like the extra noise. They are either going to make the side-panel vibrate and transmit those vibrations throughout the case, or you are going to hear some combination of motor-whine and air-turbulence.
Since noise is a subjective factor, cooling is a matter of what you're willing to live with, I can't give you absolute advice on this. I can't say "Do it this way or that way." You'll have to play around with it.