|No you should not. Disable the PSU fan and you stand a very good chance of overheating. |
Although the case is open, the PSU is still in an enclosed space and needs air to circulate through it.
In reality a case open on both sides is more likely to overheat than one that is closed with properly placed cooling fans generating an air flow through the case. If you feel the need to operate the computer with the case open there is something wrong with your cooling arrangements. Probably too many fans.