sqlrpgle select statements

I had an pgm running too long & found that one of 7 different
select statements was scanning the whole file. The fields in the 
where clause were always the same, so i created a logical. Ran again
and was not any faster. Then remembered a comment in SQL at Work book that
using the logical name in the select statement would work better, and it did.

The other 6 files have an existing logical where 
at least the first field in the where clause is the same as logical key, 
so should i change the select statement to use the best logical name, 
or leave them alone. 
I'm not sure if to only specify a lf name when there is a problem or always 
specify?

jim franz