Yes, printing a QR code on a flat surface is recommended, as it is easier for most smartphones and QR scanners to detect and read them. Not to mention, if the surface is not smooth, the QR code might end up being fragmented and divided, hindering its readability. So, placing codes on an irregular surface will cause trouble while scanning them. The surface material also affects its scannability; for example, if the QR code is printed on a reflective exterior, some scanners may have trouble scanning it. Therefore, it is necessary to follow the QR code printing guidelines and requirements.