keep in mind that if you write cron jobs, the .profile is not run when the cron job fires, so you will either have to hard code your paths or source in the .profile to each shell script you put in there.
Exporting PATH won't work. When you switch to root using the su command, your PATH is reset to a default value which is /bin:/sbin.
You can modify this default value by changing SUPATH the /etc/default/su file.