If you have a line with a mailmerge field that is blank (i.e.not even any
spaces) and with no other text on the line, Word should suppress it by
default.

If you have a blank line with no fields, or which contains spaces, Word will
not remove it.

If you have WOrd 2000 or earlier, there's an optoin in the MailMergeHelper
dialog box Merge|Options dialog to select this behaviour. If you have Word
2002 or later, the option should be on by default, but if you want to check
you can get to this dialog by going into Tools|Customize|Commands,select
category All Commands, locate MailMergeHelper in the list of commands, drag
it to a toolbar, then use it to check/set the option.

However, in Word 2002/2003, blank lines will not be suppressed in some cases
where suppression would have occurred in Word 2000 and earlier, specifically
where the field is nested within another field such as an IF field or
INCLUDETEXT field.