Is it bad form to use "LIMIT 1" on queries that should only return one row anyway?
For SELECT, I think one of the main considerations has to do with performance.

When using LIMIT 1 statistics on row correlations could play a part in determining slower/faster query execution. Although this may not be immediately observable with small data sets, it could become an issue as row numbers and index size grows (depending on DB).

Take this example, where a Postgres user encountered different execution times due to the DB's different assumptions about execution costs (based on available statistical data in the pg_statistic catalog and particularly correlation values) and therefore, resulted in different index scanning strategies.

As a rule of thumb I would use the LIMIT clause only where necessary rather than to externalize my expectations.

For UPDATE and DELETE as already mentioned, the behavior is highly DB dependent.