Oooooh, just you wait until DREW CAPSLOCK gets here. And get your earplugs in!
Anyway, you do need to get it vaguely right at exposure time if wet printing, especially for a high-contrast/high-saturation film like Ektar. Print hue-corrections affect the shadows mostly, whereas on-camera filters affect the highlights mostly. You can correct minor errors in printing so I don't personally bother with an 81A, but if the light calls for an 81C then you really want to use it. You can probably get away with using an 81B for any situation that you think might need some warming and then correct either direction from there while printing.
When you say you're new to C41, does this mean you previously shot E6 or just B&W? If you shoot E6 and can filter for that, do the same thing. C41 is more forgiving.
If you're carrying a DSLR then you can preview colour with that. Put it in Daylight white-balance and take a test shot or three with whichever filters you are considering. Inspect the RGB histograms (against your memory of a known-good histogram of a similar subject, appropriately filtered), select your filter, put it on the 4x5 and away you go! It takes quite some practise to not be deceived by the LCD image though because if the LCD is any good, the colour of the image will of course match your surroundings and seem "correct" even if it's waaaaaaay off neutral. Then you get home, look at the image in neutral light and go "oh bugger".