MX is to tell other mail servers how your domain will receive mail. If you're not doing that with your local Postfix, then you don't need to do anything with an MX record. (Organizations can have multiple MX records, but that's more for redundancy. I believe Google will have multiple MX records for your Apps account, for example).
You should have another SPF record, to tell anti-spam filters that your local Postfix is a designated sender for your domain. You may want to do more than just an SPF record, depending on the "quality" of your IP address, i.e., if it's in some dynamic pool blacklist, you will have problems with anti-spam filters.
Note that your local Postfix can send mail regardless. Having the right SPF record, etc., is to address anti-spam filters.