I nearly always fly FSX using ASE build 647 in historical mode so that I can exit the simulation and then reload it several hours later to continue/complete my flight. Is it necessary that I exit and reload ASE each time or can I simply leave Active Sky running but minimized to my task bar?

ASE can always be running! A restart is suggested when changing significant options including your depiction mode, but otherwise, no reason to shut it down. We do ask that if you are not using it, please up your download interval, there is no reason to grab new global weather every 5 minutes if you are not taking advantage of such updates. The servers will thank you :)

Sounds good Damian. Thanks PS is the new wx programming ready yet? Just kidding, couldn't resist.

After a few years I am finally beginning to understand how ASE works... ;) And when I read this question I thought that another option you could use could be 'Force to sim time': when that option is selected and you start FSX, ASE will load the weather that was there at your sim time. If I am not mistaken this is some sort of equivalent of the historial date option...! So in that case you could simply save your flight and when you load it later on (the same day or weeks later!) the Force to sim time option will make sure the right weather for the date and time you were flying is loaded too. If you know what I mean. ;) I think that simply setting your date and time in FSX is easier and more convenient than having to load specific weather manually from within ASE. EDITO, and BTW (correct me if I am wrong) another advantage of using Force to sim time over Historical weather is that with Force to sim time you can still use Automatic downloads! Afaik historical weather disables automatic downloads and so you are stuck with certain weather: Forcing to sim time will still load the weather to your set interval, but it will always be the weather from your sim time date! Huge advantage imho (and since I am rather new at all this: please correct me if I am wrong! ;) )