Cloud, I personally don't see anything TACTICALLY advantageous about carrying an underloaded mag. I can see a potential for a very slight comfort issue, but nothing tactical about it.
I'll be the guy to say it- do you have the right gun for the job? If you're gun is not something you can feel comfortable concealing, then perhaps you need to look at a different weapon.
I personally have 5 different weapons that I carry in some assortment of primary and back-up guns, ranging from full size to pocket size, and can carry in a variety of styles- IWB/ OWB/ Ankle/ Shoulder/ Pocket. I know some people suggest that you dress around your gun- I'm giving myself options, by choosing my weapon and carry style based on my dress and environment- is it hot or cold, am I wearing a jacket or shorts, etc. I have traded around a few, and found guns that I can carry effectively- and they all play a role in how I carry.
Just my humble opinion, your results may vary, IANAL, IDNSIAHIELN, and this opinion has a 1/100 cent cash value, redeemable at select locations.