> What is very bad is TeX's behaviour of linespacing the lines with
> overhanging material (like integrals, badly formatted inline
> fractions...)

If you want to keep lines on the grid even in the presence of such stuff
you can ask tex to work a bit harder, eg:

\everymath{%
  \global\setbox1\hbox\bgroup\everymath{}$\aftergroup\foo}
\def\foo{\egroup$\smash{\box1}}

\def\x{aaaa aaaa aaaa aaaa aaaa aaaa aaaa  }

\x\x\x\x\x\x
${\displaystyle{a\over b}\over {c \over d}}\over
{\displaystyle{a\over b}\over {c \over d}}$
\x\x\x\x\x\x\x

\bye


You could make this a bit smarter, eg to snap the size of the box to
an integral number of baselineskips (rather than to zero as here)
and to unbox smaller examples to re-allow linebreaking, and to worry
about mathsurround etc...

Although as Michael said, it's not clear whether if you have a lot of
such things you are ever going to get it to fit the grid in a reasonable
way.

David