Shift at whatever makes you and your wallet happy. I generally shift between 2k and 4k depending on road conditions, mood, if I feel like getting good mpgs and saving a few bucks, or if i want to actually get somewhere. I'll occasionally run it to redline because I can. Any modern engine can handle it, just don't do it on every shift or it bounce off the limiter.
Mine: 2008 Saab 9-3 Aero SportCombi XWD: 82k, Black, MANUAL
Gone: 2002 SVT, #1654
Hers: 2010 Focus SES: 70k, Ebony, Auto, Moon and Tune