If you were watching closely on Friday, Apple stock hit a new all-time high: $120. Adjusted for Apple's most recent stock split, that's $840. The new high was propelled by its record fiscal 2015 Q1 results on Tuesday, which exceeded analyst expectations. While some Apple shareholders may be tempted to take their gains with the stock up 67% in the past 12 months, investors should think twice before they cash out. Here are three reasons not to sell Apple Stock. 1. Apple's valuation is still conservative. Before Apple reported first-quarter results, the tech giant's price-to-earnings ratio with the stock at $120 would have been 18.5 -- fairly conservative for a profitable cash cow with growing revenue and earnings. But after taking into account the company's 48% quarterly EPS growth in Q1, Apple's new P/E at $120 is now 16.2. Measured by price-to-earnings ratio, therefore, the stock's 5% jump this week was definitely justified. Further, at just 16.2 times earnings, Apple stock is starting to look cheap again.