I tried it once to see if I could extend my range compares to a mates identical bike. I saved virtually nothing just a few percent and had to work a lot harder so put it back to stock settings.
I dialed the lowest setting to the lowest it will go (75%) which is my 'get me home' mode. not sure how much battery it saves tho...
I also set the 2nd mode to be closest to what the 1st was as standard. I use the 2nd setting the most for normal riding. never touched the 4&5th as they hardly get used.
All my settings are at their lowest except 4 and 5. 4 is one down and 5 is max. I've never needed 4 or 5. At full leg power and at max cadence for the situation, I feel no change between 3 and 4 or 5. If I ride at lower leg power and let the motor do the work I can feel the difference between all the modes. I found that the stock settings gave me too much assistance, as I want the bike to behave like a normal bike. The best way to save power is not necessarily to put the power lower but to ride the bike faster so that you go beyond the support level as the motor then stops draining the battery. I am 63 years old, weigh 111 kilos and ride with young guys and keep up and use less battery on Giants as well as other eBikes. Some of the lighter guys do get better milage on their Specialized Levos but that is a weight issue. How you decide to use your eBike is of course up to you. If you want the motor to do the work and have fun, that's cool. I want to have fun but still want to feel that I'm riding a bike, so I manage my heart rate and push as hard as I can, then I've got the fallback that I can use higher support if needed. So far it wasn't required except one day when I developed some severe neck spams and just wanted to get home. Don't be shy to experiment, is worth it. What I found is that the lower support makes my technical riding better in tight sections where I am negotiating switchbacks etc. When you have too much support your bike lurch forward to much and you lose control. Also your gear changes are better when you run with less sorry as you don't have the motor continuing as hard after you back off slightly for a gear change. Also when running at a higher cadence with less support you can easily change great without backing off. Initially I broke a tooth of my casette although I believe it was more due to a faulty freewheel hub then due to changing gears under power. When I ride with people with bio-bikes I often ride an entire 30 kilometer rife without the motor on otherwise it would be no fun for the other guy. I was surprised how easy it is and not much harder than on a normal bike. The problem with starting at a too high support level is the addiction factor. It's very difficult to go down on power on a trip once you started out at a high support level. On the end personal preference and fitness plays the the biggest role in how you end up adjusting your settings. The way mine is now set up give me very even jumps in support between the 5 settings.