Can you exclude issue numbers in references based on APA 6th edition?
The general rule in APA 6 is that if the journal is paginated by year, then you exclude issue number and just report volume number. From my experience, just about all journals in psychology are paginated by volume/year (and not issue number), so you can almost always exclude issue number. 

There is a general discussion here on how to determine whether a periodical is paginated by issue.

However, I honestly can't see myself going through every reference and checking whether it is from one of the rare journals that is paginated by issue. And presumably if there was occasionally a journal that happened to paginate by issue, it would not be the end of the world if the issue information was excluded from the reference. You still have authors, year, title, and so on. So, that's enough to identify the reference.