I don’t really see why you would change any gravities for VG, PG and your nic though. Their gravities are pretty stable if you have a pure product. There may be very little variations but too insignificant to change the default values of
PG: 1.036 g/ml
VG: 1.261 g/ml
If you’re going to start playing with these variables (?), others are going to start to have difficulties reproducing your recipes should you ever share them. If you create a recipe and work with the default values, you’re always going to be able to reproduce it, even if the weight is not 100% accurate and that’s what recipes are all about, the ability to reproduce your results.
This is what the site uses if you don’t set your own specific gravities. For flavors, it uses an average number but PG and VG gravities are known values that are pretty accurate if you use pharmaceutical grade VG and PG (99.9% purity). Few people will set their own gravities.
This may give slight differences when you mix a recipe from someone who uses syringes (volume) vs someone who uses scales (weight) but in general, the differences are smaller than the accuracy that most people mix with (a drop more or less, who cares). And when you do find a recipe where a drop matters, this is usually highlighted in the comments of the recipe.