Actually the LEFT(MyValueHere,6) by itself will give you the left 6 characters and does not need the ALLTRIM() added unless there might be leading Spaces in the value.
If the non-blank value of MyValueHere should be less than 6 characters, you will get the value plus a trailing number of Spaces up to a total character count of 6.
Some things to try
MyValueHere = '1234567890"
?LEFT(MyValueHere,6)
MyValueHere = '1234 "
?"|" + LEFT(MyValueHere,6) + "|"
MyValueHere = '1234"
?LEFT(MyValueHere,6)
Good Luck