Generally all pins on a microcontroller have clamping diodes for protection. That means that in practice, anything over ca. Vcc + 0.6V is clamped to Vcc. Conversely, that means that if you apply 5V to a 3.3V uC pin, the following might happen:
- Clamping diodes on the uC conduct excessive current, causing damage to the die (local overheating).
- The clamping current is sourced by the device that applies the excessive voltage, exceeding maximum ratings of said device.
- The 3.3V power supply is being forced to sink current through its positive terminal with unknown effects on the PSU.
So you tell me - risk it, or perhaps stick to the ratings of each device in the chain?