Should I upgrade?

NOT update, there are too many problems with many people who got 9.33, better wait it out and DISABLE auto update, until the problems seems solved about battery draining.

6.57 looks like a firmware older than the first official ones that came after the launch of the Epix 2.

You will, probably, need to update as that firmware could be an "incomplete" one.

If everything is working well...then it's up to you of course

No, when its not necessary i wouldnt. theres definitely something wrong with the new fw. luckely i dont see a battery drain wirh this version, but other problems like laggy ui, map screen during workout sometimes blank, specially in citys where you have a lot of details in the map, message that ext hr monitor was connected not always showing up...

If you use the watch mainly for simple fitness/activity tracking, you don't mind the occasional crash, you don't need Garmin running power, you don't obsess about training optimization or you absolutely need long battery capacity for some upcoming day-long+ event, you are fine with the older version. Use Garmin Coach for your training needs up to Half M.

If you are preparing for a marathon, looking forward to training optimization metrics (let's say you are just fed up to have too many unproductive workouts :-), you can't bear some pesky cosmetic (all things being relative) older software issues, or you are OK with charging your watch every couple of days in the case you get the battery drain flu, then upgrade to 9.33. 

Also, don't stress over the upgrade. It is just a sports watch. Stress's bad for your training readiness whether you can measure it or not :-)

I definitely don't agree with the previous comments. 
For me, YES, you should definitely upgrade your watch, especially if it's brand new. 

First, if you have a look a the changelog since 6.57, you'll see that the number of bugs smashed by the dev teams is really impressive and outnumber by far the bugs experienced by some users with the 9.33 firmware.

Second, the battery drains is not a general issue experienced by millions of users... and is quite specific to some use cases. (see the official topic in these forums for more). Personally, I've update my EPIX 2 on day one, I train 10-12 hours per week (it means using the following activities on a weekly basis at the very least : yoga, run, trail running, road bike, mountain bike, ebike, indoor bike, weight lifting, hiking and walking), I use advanced features such as workouts, pacepro plans and navigation. I wear my EPIX 24/24 with sleep tracking and all-time on display...

To be fair, the only feature I've never used is the music player (I still do not think it's the right use case for this watch... Apple Watch was built to be a music/multimedia watch, not Garmin's ... but that's a personal point of view).

That being said, I did not experience a single bug or crash with the new FW. Quite the contrary, the previous versions included some annoying UX bugs that are now solved. Your watch is new, not a lot of datas or settings that could cause conflict with the latest changes. I would recommend to update it as soon as possible before using the watch. 

That's my personal feeling based on my personal (and professional) experience based on such systems.

I concur. There will always be problems that some experience and others don't. There are few problems that affect everyone. Forums such as these lean heavily towards the negative as, generally speaking, people with problems will post a lot more than people without. 

I'd worry less about what people post in these forums and focus on getting the best out of the watch. It's a great watch and leaps and bounds ahead of other brands.