It is better not to do any maintenance on the PLCs other than blowering to remove the dust accumulated on the plc modules.
Please remember to switch off power to the racks before doing any maintenance.
From my personal experience, i will recommend that you SHOULD NOT remove/unplug the modules from the racks especially in case of S7-400 / S5 racks.
These racks are very sensitive to dust and the "maintenance" might result into unforeseen problems due to dust ingress.
Other than the above, checking tightness of mounting screws and a regular backup of online data always helps!