On pfSense 2.0.1 and earlier, the log files were always wiped/reset at bootup.
On pfSense 2.0.2 and later, on a full install the logs are kept at bootup.
On NanoBSD, the logs are kept in RAM and would be wiped after each reboot no matter what.
If you need to keep logs indefinitely, setup a syslog server and have pfSense send its logs there.