Use the drops before smoking.
While high, sometimes my friends would get too paranoid to even use drops thinking it was some other chemical and they'd be blind forever if they used it (even though the label clearly says 'eyedrops').
Also, I find that drops keep me from the lazy/tired feeling. Red eyes make you more tired.
Also- what if someone catches you before you get a chance to even get your drops? If you use drops before hand, you'll never get red eyes in the first place.
Also- using drops before hand makes you less paranoid and feel safer. You might even forget or be too lazy to use drops after getting high (happened to me too many times)