PIC I/O pin status on power up
PIC I/O pins wake up in high impedance state on power up.

The reason you see the LED glitch is probably because the 5 V supply is coming up faster than the 3.3 V supply, and the LED cathode is temporarily held low thru the protection diode on that pin in the PIC.  This is not good, and can lead to latchup on power up.

If you can arrange to be sure the 3.3 V supply comes up before and goes down after the 5 V supply, then your scheme can work.  However, it is better to use a transistor, or to run the LED from the 3.3 V supply.  The latter will also waste less power, assuming your 3.3 V supply is reasonably efficient.