Do investors need to be worried about Micron's sell-off today?Micron (MU -3.63%) stock got hit with a valuation retracement in Friday's trading. The company's share price fell 3.6% in the day's trading, and had been off as much as 5.6% earlier in the session. Meanwhile, the S&P 500 (^GSPC 0.49%) gained 0.5% in the session, and the Nasdaq Composite (^IXIC 0.72%) rose 0.7%.Micron stock lost ground today despite gains for the broader market and little in the way of clear-cut, company-specific news. Even with t ...