I am using Heroku for my site hosting, and Zerigo for DNS magagement. I'm trying to setup Google Apps email, but I can't add the last MX record (ASPMX5.GOOGLEMAIL.COM.) because I have hit my limit of 10 allowed total records on Zerigo. Will it matter if I don't add this last record?
No it won't. Google Apps gives you that many MX servers for heavy redundancy in case of problems. MX records have a server to send attempt to send mail to and a priority associated with it (IE which server are you in line to receive mail for this domain if the other isn't available). You weren't able to add the lowest priority server to your DNS. Normally no mail server would attempt to connect to it (aspmx5.googlemail.com) unless all the others were unavailable. In an instance like that we'd probably have bigger problems to worry about :)