>I want to add sounds to a product--beeps and boops and the like.
>
>Can I just hook a piezo speaker to a PIC output pin?
>Or should I use a resistor and/or capacitor in series?

Yes but.

You'll be tempted to use two outputs to make it louder. This can, with
the wrong switching sequence, cause a voltage doubling effect, and
hang your processor with CMOS latchup!

Wrong sequence.

A  B      Voltage on pins at the moment of transition

L  L        0     0
L  H        0     5
H  H       5     10 (!)  Possible CPU hang.
H  L       5      0
L  L        0     0


Right sequence

A B

L L     0  0
L H     0  5
L L     0   0 (This state just has to happen, it dosen't have to stay there
any significant time)
H L     5  0
L L      0  0 (Same as above)