Slackware's sample smb.conf file contains the following line:
log file = /var/log/samba.%m
Now there's two log files log.nmbd and log.smbd that are stored anyway in /var/log/samba, and as far as I understand, this cannot be altered.
For consistency's sake, I suggest the following in the sample smb.conf:
log file = /var/log/samba/log.%m
My daily two cents