If you are talking about JOIN condition, it is better and more correct in the JOIN clause, not in the WHERE clause. It is also better to limit your result as soon as possible. Consider this:

SELECT col1, col2

FROM t1

JOIN t2

ON t1.c3 = @variable

AND t1.c4 = t2.c5

Having ON t1.c3 = @variable before the join criteria helps to limit the result set before joining. This does not work though for LEFT JOIN. In that case you are forced to go the following way:

SELECT col1, col2

FROM t1

LEFT JOIN t2

ON t1.c4 = t2.c5

AND t1.c3 = @variable