How important is it to change the serial number for every db.xxx.yyy record. Can I keep the serial number the same for at least 2 records.
No. You must change the serial number every time you make a change, and it must be a higher number than the previous one. This ensures proper propagation and loading for slave zones.
I recommend using a ten-digit serial, using the date (in YYYYMMDD format). Today would be 2011123000. Next revision today would be 2011123001. This allows for 99 revisions. The serial number will increment naturally using this method.
If you don't change the serial number, by the next zone transfer, the changes you've made will not be recognized (as the entire zone will be recognized as unchanged), and thus you might end up getting the updated records overwritten.
But it's only important if you want to maintain the changes you've made, if you just made them for fun, I'd say it's not important :-)
Please see the SOA notes here:
While technically the serial number should be updated, I find many people do not actually have multiple nameservers. Instead, all nameservers run from the same server. In this case, the serial number does not matter since the primary and secondary server are one in the same.
I however prefer to update the serial number using the YYYYMMDD date format as suggested by lunchmeat317 as this makes it easy to spot the last time you updated DNS.
… otherwise it's just frippery that nothing uses. If you don't have multiple content DNS servers, you aren't replicating your database anywhere. And if you are not using the "zone transfer" database replication mechanism, you very probably aren't using a mechanism that relies on anything in that entire resource record set in the first place.
I suspect that you are using "zone transfer" and have multiple content DNS servers. But that's just an educated guess on my part, since you don't say in your question. If so, remember that a change can comprise multiple resource records in one go. Serial number updates are tied to changes, not to single resource records.
SOAresource record. Frequently Given Answers.