Seat height

It is down to rider preference as Supersonic says, they are loads of Articles on the net about what is the "right " height and how to maximise pedalling efficency blah blah blah.

For me - I start with it quite high while I'm on my way to the trails and then just drop it down when I start doing the down hill trails.

I never have a specific height I always put it at, I just drop it and sit on it and if it feels right then for me it's spot on.

Just remember that if you start taking decent steep drop offs or similar you will need to start getting your backside over the back of the bike and a lower saddle will help you do that

A quick release for your seat is really important as you will be moving it far more often than on your road bikes.
I stick my seat up into the optimum pedaling position if there is a long climb or fire road section ahead, drop it a couple of inches for levelish trail sections where I am in and out of the saddle and drop it even lower for long downhill sections. Not too low though, you don't want to slam too hard on it if you slip a pedal!

30.9? WHat is your bike? Specialized?

Seat clamp not 30.9 then - that's the seat post size.

Is 34.9mm

For general XC, most experts suggest lowering your saddle by 10mm from your normal road height. it means you sit into the bike more and can move your bodyweight around more easily, apparently it even helps with your climbing.

I'm predominantly a mountain biker but do a fair amount of road miles for training, commuting and fun. If I've spent a long time on my road bike and jump on my MTB it feels really weird having a lower saddle so I tend to run mine only a tiny bit lower (3-5mm) but it's still a very noticeable drop. Because I'm so sensitive to small height adjustments I use a bolt-up seatpost clamp and resist the temptation to drop the saddle for descents. When I first started riding I was moving the saddle up and down all the time, but as I've got more confident and my legs have become so attuned to pedalling I find no benefit in slamming the saddle. If you're a fit roadie type I'd say leave it up but do make measurements from the pedal at it's furthest point from the saddle as MTB's often have longer cranks than road bikes. Also bear in mind that on cold days when you're wearing lots of layers (thick padded bibs + tights + baggies) you may need to drop the saddle, sounds daft but happened to me yesterday!

I never adjust my saddle height i fail to ever see the need.

For XC riding theres absolutely no need to adjust the height