I personally use MAPQ 20 for standard tasks like ATAC-seq or ChIP-seq. But don't ask why 20 and not 25, 15 or 10. I think I stumbled over 20 which was used by a reputable tool when I got started with NGS so I took it over. Unless you are investigating difficult genomic regions specifically, choice should not matter too much as long as it is > 0 to avoid multimappers.

The ENCODE pipeline uses a MAPQ 30 cut-off