@denverblueeyes - Congratulations!
It's fine to run your machine 24/7. I do it now frequently with the SRX, and used to do it with my previous E2 as well. Presumably, it is fine to run the machine for extended periods on any speed, as the 90 minute timer works on all 4 speed settings. I often set mine on speed 3 for an hour or so, or a few hours on speed 2. It'll clean up the air nicely after burning food, etc.
A couple things to keep in mind:
1. Watch the water level on the lowest speed. Depending on where you live and your relative humidity, it is likely that you'll have to add water a couple times a day. If it gets too low, the incoming air hardly hits the water and there's no splashing onto the the separator. You're basically just filtering via the HEPA filter at that point. If I'm letting mine run 24/7 I will overfill the basin a little so it will run longer. Just don't do that on the higher speed settings or the machine might flag the error light.
2. Always start with a clean basin when running the Rainbow as a filter, especially on the lowest speed.
3. Be careful with the timer. One of the biggest mistakes you can make with your Rainbow is storing it on a wet/damp basin- it will rust out the motor bearings. If you run the machine on high for 90 minutes, and then it shuts itself off, you don't want the hot motor sitting on a basin full of water for very long. I don't use the timer feature at all; I'm concerned I'd forget about it and it would sit there way to long.
Enjoy your new Rainbow!