The naming of files is just to your personally taste however from what I can tell in your filenames your not using a rotate and therefor I'd imagine that your logs are rather large and hard to shift though.
You should look at using a rotate by X hours, or days which then changes the labeling of your filenames with the date on the front or end of the filenames.
A typical example would look something like this
LogLevel warn
LogFormat "%h %l %u %t \"%r\" %>s %b" common
LogFormat "%{Referer}i -> %U" referer
LogFormat "%{User-agent}i" agent
ErrorLog "|/usr/sbin/rotatelogs -l /etc/httpd/logs/error_log.%Y-%m-%d-%H 3600"
CustomLog "|/usr/sbin/rotatelogs -l /etc/httpd/logs/access_log.%Y-%m-%d-%H 3600" combined
So personally I'd use:
date_subdomain_maindomain_error_log
date_subdomain_maindomain_access_log
Which will look like:
29-01-2013-1600_subdomain_maindomain_error_log
30-01-2013-1600_subdomain_maindomain_error_log
Or you could simply have your logs rotate every 24 hours with just the day using
CustomLog "|/usr/sbin/rotatelogs -l /etc/httpd/logs/access_log.%d"
28_subdomain_maindomain_error_log
29_subdomain_maindomain_error_log
30_subdomain_maindomain_error_log
Using the above just makes it easier to administrate and source problems faster, but again file-naming is down to personal taste and realistically there is no better answer than your own :P
To find out more about rotating logs check out: rotatelogs