Is PETG filament food safe? I've been trying to find a 3D printer filament which would not release any chemicals if in contact with heated water for a substantial amount of time. So far, I've easily ruled out both PLA and ABS, as they're not considered food safe from what I can find. I have found PETG filament, which seems to be food safe. 

My question is: "Is there's anything special you'd have to do to make sure the print is food safe, or as in my case, to make sure it's safe for usage in a mug?".

I will be using a steel extruder as brass ones may contain lead.
Many manufactures list their filaments as being food safe, but I would not treat this as "gospel truth". Apparently, the FDA considers PETG to be safe for food contact, but they are probably thinking about injection-moulded and vacuum-formed parts. Unfortunately, an initial search of the FDA's website did not yield any definitive information.

Even if a particular filament is genuinely food safe, that does not mean that a 3D-printed part made from it will be food safe, since there will be an abundance of nooks and crannies where bacteria can lodge and reproduce. You would have to sterilise a utensil before and after every use to be absolutely safe.

Anyway, good luck with making a water-tight mug with an FDM printer. You will probably have to seal it to make it water-tight, and then it will be the food-safety of the sealant that you will need to worry about. I would give it a miss, if I were you (at least, for other people's use). Items intended for one-time use would be OK, I suppose.