They stopped doing that some time ago, though I can’t remember when exactly. The reasoning was that people would realize five minutes into the hour/half hour that there was nothing good on, then turn to TBS because the show there was just starting and they hadn’t missed anything. Furthermore, once you start watching +5 minute style TBS, you are off schedule to the other stations and may be less likely to change the channel cause you can never catch the beginning of another show if you stay to the end of a TBS show.
Nowadays the networks are starting shows at off times to throw off Tivo and to counter programming on other stations…for example, NBC regularly starts ER at 9:59, thus people (like me) who watch the previews for next week’s CSI miss the recap & often the start of ER. Check out the weekly Nielsen listings in USA Today to see all the oddball start times the networks use now.