Shower tile - verticle seams - caulk or grout?

To get an answer to this question, I of course first used a google search, which offered up this whopping gold mine:
http://groups.google.com/groups?q=%22caulk%22%20%22corner%20joints%22%20sho wer&hl=en&lr=&ie=UTF-8&sa=N&as_qdr=all&tab=wg
Okay, so I did some more searching on the web as well, but basically, the picture I get is that this has been a divided issue.
Not much of a sampling to get any clear picture though.
Every place I have lived here in the Ohio Valley has used grout in the verticle corner seams of a tiled shower, yet the guy at Home Depot said to use caulk. Some on this group have said to use caulk. Others, grout. Has anything improved on the grout front in recent years that might change any opinions? Also, I'm doing a repair job of just a portion of the shower wall, though it does extend to the corner joint. If I were to grout, could I just grout over the old grout that remains in the portions that didn't require repair (and which seems in good shape)? Or should I knock out all the old grout and then caulk? Finally, how about leaving the old grout in the untouched portion of the verticle joint, grouting the repair portion (sanded since it has a large fill area behind it), letting it cure for a week or so (perhaps hanging plastic over it when shower is in use), and then going back and caulking over it?
Hell, I don't know. Chime in.
Thanks.
Oh, p.s.: This shower is in a rental unit. The porcelin or enamel or whatever it is on the tub is worn and its beginning to get rust. The whole thing needs to be torn out and replace with a wall unit, but I would have to tear out walls around it to get the unit in. So really I just need something to hang on for three or four years until I can remodel the whole bathroom.
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Just wanted to clarify my subject header

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cripes, I see another mistake:

should read, "...could I just *caulk* over the old grout..."
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Corners have a tendency to crack, that's the reason for the caulk. You don't want to go over old material with whichever you decide to use though.
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