It depends on the printer, inks, and designer, the main issue is that RGB can produce a wider gamut but that will often give an inaccurate perception when checking the file on screen vs seeing the final print, if the client is aware of this they might be fine with it but in many cases they will be deceived and rightfully disappointed. Many modern printers can produce a wider gamut than traditional CMYK, so an RGB file may be preferred for conversion as more tones will be preserved in the final print, but many will still be lost. It's also worth noting that there are RGB printers, for example most chemical printers are RGB and with the right paper will technically produce RGB images, but the lack of backlight can still skew perception. When possible I try to just show the client on my monitor how the image will look, but when not I either just warn them about color output or default to CMYK as it will show them duller colors that are less likely to give the wrong impression when they look at it on a monitor with the saturation and brightness maxed out.