is it helpful for the optimiser/planner to add LIMIT 1

I'm reviewing some function I wrote to add stable, immutable where
needed and I'd like to take the chance to add further "cheap"
optimisation if it helps.

There are many places where I know a function or a statement will
return just one row?

Is it helpful to add LIMIT 1?

eg.
select a, b from myfunction(3,5) limit 1;
select into a,b x,y from tablename where z=5 and u=7 limit 1;
select a,b from from tablename where z=5 and u=7 limit 1;

thx

--
Ivan Sergio Borgonovo
http://www.webthatworks.it