Is it okay to have no A records for a domain?

Yes, it is not a violation of any rule to not have an A record for a domain (none of my domains have an A RR).

You should be covered by your mx records and cname.

Mail Exchange records direct email to servers for a domain, and are listed in order of priority. If mail can't be delivered using the first priority record, the second priority record is used, and so on.

"A records (also known as host records) are the central records of DNS. These records link a domain, or subdomain, to an IP address.

A records and IP addresses do not necessarily match on a one-to-one basis. Many A records correspond to a single IP address, where one machine can serve many web sites. Alternatively, a single A record may correspond to many IP addresses. This can facilitate fault tolerance and load distribution, and allows a site to move its physical location.

Google Apps does not support IP addresses alone. Instead of using A records, you can set up email and web publishing by modifying your MX and CNAME records with your domain host."