It's a requirement for any switch that does IGMP snooping, Juniper or not. If IGMP snooping is in play every broadcast domain that expects to have multicast flowing needs to have some device send out IGMP queries. IGMP snooping must see the report from the device expressing interest in a given group before data for that group will be passed on.
The only exception is if the destination address is 224.0.0.x. That block is special and should pretty much always be forwarded regardless of whether or not IGMP snooping is in play.
This actually bit me at home once because a wifi router I had handling some traffic did igmp snooping but there was no mention of it anywhere. I had a device attached to it and was trying to do stuff with DLNA. If I completely power cycled the device I could get DLNA to work due to the gratutious IGMP report sent when the device powered up, but a few minutes later it stopped working when the entry expired out of the IGMP snooping table. I had to turn on IGMP so that there was a querier on the network. The IGMP query goes out, the device sends its IGMP report for what it is interested in, and the IGMP snooping table is updated to reflect that the device is still interested.
Also, there should only be one IGMP querier in a given broadcast domain. If you only have one switch, then that's where you'd configure it. If you have multiple switches, just pick one to act as the querier; don't enable the snooping querier functionality on all of them.
-Chad