As you suggest, if in doubt using the body copy’s leading is always a good starting point. I find myself using one of three approaches:
A baseline grid based on body copy leading - the easiest to work with. Smaller and larger type can be anchored to the grid and will ideally have leading that is sympathetic to the baseline grid.
A baseline grid that is a fraction of the body copy leading - this is useful where I’ve got a lot going on. I used this recently within a document that had a lot of images and figures/captions. In this case it helped keep things neat and easy to work with.
A baseline grid that fits the wider page grid - ideally they all will. In practice this can often be harder than it sounds. (I’ve often had baseline grids that are a point or two (or much more!) out when I get to the foot).
Which one you choose depends on what will make your life easier rather than something that seems right but is a pain to use. What works also varies from document to document. The key point to remember is that no one will ever see your baseline grid (directly). They will see the impact it has on your work - which is, ideally, rhythm and consistency.
[EDIT: You also asked about headings and subheads relative to baseline grid. I’ll usually set up a heirarchy of heading sizes (and caption, etc) sizes relative to the body using a clear scale. That means that their leading will likely be a multiple of the body’s. This means that a heading might sit on the baseline grid every 3 lines, etc.
Your question here probably relates to the space between a heading and body copy. This is easy when all headings are one line but more complex when they are multi-line and vary throughout a document. There are a handful of ways of working with this. Again, which is right depends on the work you are producing. Basically:
- you can make the heading leading match the baseline (or be a clear multiple of it - 2x, 3x, etc)
- you can set up a default heading area (or areas) that is always clear of the body.
- you can fudge it and add enough space around the headings that people won’t notice a part-line more or less around some headings.
Probably not quite as clean an answer as you were hoping for, but it’s challenges like this that (knowingly or not) people hire us to solve]