I believe you should. You don't have to, but if you don't the low G will sit too high and may buzz or lose tone.
I have these great sets of needle files I found at a woodworking store, it consists of a handle and several files, round, 3 corner, square, flat, etc. All the files fit into the handle. The smallest round one is perfect for a low G.
If you don't have something like this, you can take some sandpaper and fold it over a playing card, and sand the slot. You might be able to wrap the sandpaper in a round toothpick and go back and forth. It would be best if the slot was round shaped, like the string. Try not to go any deeper, just enlarge the width of the slot. You will still be able to go back to high G. Smooth any sharp edges, especially at the mouth of the slot.
One final note, low strings do require a slightly higher action because the width of their wavelength is wider. So don't go too low. But most ukes have high actions to start with.