Headings on Baseline grid

A baseline grid is an excellent use of the facility for a book (novel/biography style).

Use Paragraph Styles throughout and (depending on the design), perhaps a full-one for the first paragraph and an indented one for subsequent paragraphs.
I suggest for your 12/18 body text you set the baseline grid to 9pt so that you align every other line for your text and it will give you more flexibility for space around subheadings and similar. Headings can be multiples of 9pt (e.g. 18pt).

If a style is set to use the Baseline grid, it will snap to the first grid line that is less than the style's total height. Unless your heading is less than (or equal to) 18 points in leading/line spacing and space above, it will skip a grid line (or two, or three) to snap to the first one it "fits." (Space below will apply to following text, of course, forcing it down in one-line increments as needed.)

Note also that you can define a different height/starting location for the first baseline in each text box, so if your pages always start with a heading, you can adjust that first baseline to always fit the heading precisely. (There are also additional twiddles to this setting so that you can position the first line by various typeface dimensions.)

Using a baseline grid is a fairly advanced technique with a lot of subtle aspects and needs. The simple asnwer is not to use it unless absolutely necessary, and IME it's rarely necessary and not always an asset for a layout.