setting up environment variables

[Dom <debian.dom@gmail.com>]

Hi, 

Could someone please tell me if I'm doing this right and if not what
I'm doing wrong as I'm a newbie trying to get acquainted with the
linux system. I want to make every application that wishes to use my
/tmp folder on my linux partition to use a custom folder on another
partition (looks like I made my linux partition too small for my needs
and need to free some space now) and searching on the internet for the
solution it's not that easy to tell what is right and I don't want to
damage my system by doing something wrong.

First I saw a document that suggested to change some 'tempfile.py' and
I did that but it didn't work as I wanted it to work.
And I am almost positive that I know why it didn't. (I have only
changed the default locations of tmp folders that IT would check AFTER
checking environment variables TMPDIR, TEMP and TMP...)
SO I need to change system variables TMPDIR, TEMP and TMP... how
should I do that?
One source tells me to change .bash_profile from my (and every other
user's if I want to do this as the system default) home folder by
putting 'export TMPDIR=[path]' in it, BUT
another source tells me that /etc/profile, ~/.profile, ~/.bashrc and
~/.bash_logout are standard scripts that are executed at various times
and  ~/.bashrc is executed when bash is launched.
So now I'm not sure which one to change, .bashrc or .bash_profile...
Can somebody tell me?

BTW, is this the only way of setting system default variables or I
missed something?

Thanks in advance,
Dom