I picked up a brand new Tikka T3 Lite, .270 WIN and since then, have fired about 150 rounds using relatively inexpensive factory ammo, including Winchester Super X, Federal, Fusion (Federal) and Sellier & Bellot.

A week ago, I picked up a copy of C. Rodney James' "The ABCs of Reloading" and started reading about excessive headspacing and learned that if stretch marks/cracks appear on a fired case right in front of the head of the case, it is an indication of excessive headspace.  See Photo 1

Armed (excuse the pun) with this knowledge, I examined the fired casings from my rifle and became somewhat concerned at what I found because most, if not, all of the fired casings had a shiny ring in front of the head. Photo 2, 3 & 4. The unfired cases do not have this shiny ring in the front of the head. So far, none of the cases have separated or cracked.

Are the rings an indication of case stretching and if so, is it sufficient to be of such concern that I should have the headspace checked by a gunsmith?

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Your fired cases look OK to me. It's normal for cases to expand slightly to fit the chamber after firing. You should start to worry when there are CRACKS around the base of the fired brass.
If you have this much once-fired brass, and are worried about case separation, I would just reload each case twice. More than that, and you're probably going to need to trim the brass anyway. You'll know when the brass needs trimming when the cartridge won't chamber easily.
I would worry about the S&B brass separating more then the U.S.-made brass (maybe reload this just once), but that's just me.

Looks pretty normal to me.  With XS headspace, there's usually a quite bright ring back there resulting from the case grabbing the chamber and the head setting back, thereby stretching the brass at that point.  I would respectfully disagree with "You should start to worry when there are CRACKS around the base of the fired brass."  The time to worry about it is before there are cracks, although modern firearms are pretty good about venting the gas away from the shooter's face.

Check your case length after firing. About a 0.005in lengthening is about maximum for one shot, if I recall correctly.  This type of normal stretching mostly occurs in the neck of the case.  Again, quite normal, but this type of stretching  is why you use case length trimmers when you reload.

Nominal unfired .270 Win case length is 2.540 in (64.5 mm).

Hatcher describes some experiments with excessive headspace and it took quite a bit to actually get "dangerous," although velocity suffered.  These experiments involved reaming out the chamber to a 1/10th inch excessive headspace in the .30-06. (See pp 250-252 in the chapter on Headspace, Hatcher's Notebook.) 

This is a very interesting chapter, by the way.

But in the spirit of the "Don't run with scissors" meme, always wear shooting glasses anyhow.

Terry, 230RN

Edited to corect BBS code.

Buy a set of Go-No Go gauges for your rifle.  They will tell you for certain if your chamber is within spec.

If you're really a go getter, there is molding material that can be heated and poured into your chamber to give you a positive of what it looks like.

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Looks normal out of a roomy factory chamber. 

I think it is more of a function of the diameter of the chamber is a little on the large side, as opposed to excessive headspace.

If you reload, when you full length size, those bulges will iron right out.  My remmy 700 in 30-06 produces similar results, and properly sized and reloaded cases have been reused 4 or more times.  With the 'looseness' of factory chambers I hesitate to reload the same case more than 5 times though.

You can use Cerrobend (or another low-melting alloy like Wood's Metal) to cast the chamber, or molten sulphur.  But I don't think it will tell you about the headspace exactly because you have to measure to an imaginary datum line on the case shoulder. 

It will tell you if your chamber is a little oversized, however, since these measurements are direct.

Chamber casting metal is available from Midway, among other places.

http://www.midwayusa.com/product/462291/cerrosafe-chamber-casting-alloy-1-2-lb