You only modify your mail server if you want to check SPF protected mail coming in (block unauthorized MX senders) otherwise all your doing in the DNS is telling everyone who your mail senders for your domain is.
here is mine as an example which uses version 1 SPF and just says that only our MX records servers are the only ones who send mail out as companydomain.com.au, otherwise block the email if you get something from elsewhere.
; TXT . SPF records
companydomain.com.au. IN TXT "v=spf1 a mx -all"
for my server to check mail coming in, my /etc/postfix/master.cf has these lines.
policy unix - n n - - spawn
user=nobody argv=/usr/bin/perl /usr/libexec/postfix/smtpd-policy.pl
my main.conf has this added to the section of 'smtpd_recipient_restrictions'
and of course "/usr/libexec/postfix/smtpd-policy.pl" must be installed.
hope this clears things up a little.