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: 2002 Focus SVT: 142k, Sonic Blue, #1654, All Options
Mods: Tom's Tune, Borla, HFC, STS, Painted Machs, Euro Grill, FRPP Springs
Hers: 2010 Focus SES: 67k, Ebony, Auto, Moon and Tune