TCM tries to show all films in their original aspect ratio, but some obscure films haven't had WS transfers, and some films that have had OAR WS transfers still show up in 4x3. But, most films that were made in WS are shown in their OAR.

On TCM-SD, that means they are letterboxed. On TCMHD, they are shown WS, so they will fill your screen (side-to-side, at least,) without having to be zoomed. Of course, 2.35:1 films will have black bars at top and bottom. The rare 1.85:1 film that is actually shown at 1.85:1, (not 16x9, aka 1.78:1) will have very thin black strips top and bottom, if your TV doesn't have too much overscan.

But, TCM still isn't showing any true HD yet. They are uprezzed to HD. So, they usually look as good as an anamorphic DVD, but aren't HD. Speculation is that TCM doesn't have access to enough of its lineup in HD to start HD transmission yet. Me, I think they should be showing what there is available in HD, in HD.