>When an application pool is scheduled to recycle at 06h00, if this is a 24/7 server, I can see that each instance of each related worker processes are restarting exactly at 06h00. But, when the server doesn't receive much hit or no hit during that timeframe, the recycle will only happen on the next hit. As a hit will trigger this, this is not done automatically in the background if there is no activity. So, basically, this means that the the hit triggering a recycle of the worker process will take a little bit longer. As oppose to being done at 06h00 on a 24/7 server, on the other non busy server during that time, the next hit could happen around 07h00 or 08h00, for example, and only at that time the related hit will take a little bit longer.
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>In order to avoid that, is there anyone of you forcing a Web hit or several hits at recycling time just so to assure that each worker process are recycling at the same time?

If this is IIS 7 then there is a 'warm start' option that will automatically fire up the app. If IIS6 then, AFAIK, there is no simple way. You could implement a Windows service (either on the web server or another machine) to generate a dummy hit at the appropriate time - but still no guarantee that a genuine hit would not occur in the interval between the scheduled recycling and the time your hit took place. If you wanted to get fancy I guess you could fire off some signal when the old version was shutting down and use that to trigger to hit.
Other than that.........