leaving out paramTypes parameter

Hi,

from the documentation, here's PQexecParams prototype

PGresult *PQexecParams(PGconn *conn,
const char *command,
int nParams,
const Oid *paramTypes,
const char * const *paramValues,
const int *paramLengths,
const int *paramFormats,
int resultFormat);

and PQprepare's

PGresult *PQprepare(PGconn *conn,
const char *stmtName,
const char *query,
int nParams,
const Oid *paramTypes);

they both require "paramTypes" parameter, but documentation says "If
paramTypes is NULL, or any particular element in the array is zero,
the server assigns a data type to the parameter symbol in the same way
it would do for an untyped literal string."

How reliable is that? Can we really leave out this parameter anytime?
What are known issues? Or should we try at all costs to specify
parameters type?

Thanks

Charles