In InDesign when I set up a DIN A4 document, margins are usually set at 12,7mm. I was wondering if this is for a reason and if there are any rules / best practices for margins as to whether one should use a certain width / height or if this is completely up to me, since I would rather like to work with 10mm as it better aligns with my document grid.
No rules, this is up to the designer. The 12.7mm is a default (not sure why really), however I have designed many printed items and always set custom margins. Many times I also used 10mm since I like to work with simple numbers. For a larger page count and if using "facing pages" you could consider a larger inner (inside) margin, for example make all sides 10mm except the inner margin which could be 15-20mm.