Yes, it is perfectly alright to keep magazines loaded. One worry is that the spring will wear out due to continuous compression. Compression does not wear out springs. Constant compression and constant expansion are what wear springs out. You will wear them out far faster by shooting them than by keeping them loaded. In fact, AmmoToGo recently did some testing that showed that a PMAG's spring would take 80 years to wear out. Polymer magazines such as a Magpul Pmag do however have a tendency to expand over time (mostly in the feed lip region) this is why their Gen 3 magazines come with a cover for the feed lips for extra support.