Brian Based on Tore answer this code shows FULL optimisation in Both cases (VFP9):

SET ANSI OFF
SET EXACT ON
CLEAR
* Turn rushmore reporting on
SYS(3054,2)
* Create a test cursor
CREATE CURSOR test (field C(10))
INDEX on field TAG field
INDEX on PADR(ALLTRIM(field),5) TAG field_t
* Put some data in the cursor
FOR i = 1 TO 10000
 INSERT INTO test VALUES (SYS(2015))
ENDFOR

* Determine the last value
cValue = LEFT(test.field,LEN(ALLTRIM(test.field))-1)
* Try the untrimmed index
?"This one works"
?"---------------------------------"
SELECT * FROM test WHERE field = cValue INTO CURSOR index
* Try the trimmed index
?
?"This one does not work"
?"---------------------------------"
SELECT * FROM test WHERE PADR(ALLTRIM(field),5) = cValue INTO CURSOR noindex