I've been weight mixing for a couple of years now. I am quite anal about it though. I measure the weight of a fixed volume of all my flavours when i first get a new one. They can vary from 0.8g/ml to around 1.2g depending on the consituents. Most are fairly close to the SG of PG (most flavours are PG based) though but some are alcohol based, thus lower. There are some VG flavours out there though and they're getting a bit more common, catering to those who like to impress people on the street and waste juice chasing clouds as well as for those with PG allergies/intolerance.
I use 1.036g/ml for PG and PG nic and 1.261g/ml for PG in my calculations and the measured value for the concentrates.
If you're using AG (VG+water), the SG will be lower than 1.261.
As said before though, it doesn't really matter whether you use volume, weight or even drops (assuming same dropper). As long as you're consistent from mix to mix, you'll get the same results each time.