Presets for things like EQ and Compression can offer useful starting points, but with some of them you have to make sure you adjust for level (e.g. the threshold setting for compression) otherwise they won’t work as they should.
If you are using an all-in-one mastering package like Izotope Ozone I think the presets can be really quite misleading as the whole point of mastering is to adjust everything to suit the specific track and it’s unlikely that any given preset will be right. I got quite confused about this when first using Izotope!
In terms of getting a better understanding of the mixing/mastering process I found this website a really valuable resource: