Should I encourage foreign students to adopt local handwriting customs?
In ordinary text, this might help a little but probably doesn't matter much, since there's plenty of redundancy to aid in understanding.  However, it can make a big difference for equations.

If the students are in a math-intensive field, I would definitely point this out, but in a broader context.  When I teach beginning American undergraduates in mathematics, I offer them handwriting advice for equations: make sure t doesn't look like +, z doesn't look like 2, the letter l doesn't look like the number 1, etc.  Furthermore, mathematics students have to learn how to write all sorts of potentially unfamiliar symbols as they progress in their studies (aleph, Weierstrass P, fraktur letters for Lie theory, etc.).  If you are working on a calculation on a whiteboard with someone, it's a real issue if they can't easily and reliably parse your equations, so just about every mathematician has put at least a little thought into this.

From this perspective, I wouldn't tell people "You're in America now and have to write like the Americans", but rather "Writing equations clearly is something everyone has to deal with.  Here are some issues you may need to adapt to, some of which are generic and others of which are specific to your background."