You certainly did the right thing by getting things as close to level as possible. The danger was not that the stand would break or collapse, it was that the pressures on the glass itself get out of balance the more that a tank is not level. There is no rule about how far any tank might be allowed to get out of level before it becomes a risk.
A tank out of level changes the stresses on the glass and the seals. However, this problem can take some time for it to actually cause a things where the tank can leak or, if they are bad enough, the glass can actualy shatter. I had an online friend with over 50 years in the pet and fish business who was backing out or the hobby. He had a single 135 gal. tank in his living room. One day the front glass gave out and he had 135 gal.s of water and flopping fsh all over the place.
I learned my lesson about this on the cheap. I was putting a small 5.5 gal tank on the bottom of a stance. For some reason I had it placed front to pback while i was getting it set. I had a couple of inches of water in the tank and started to rotate it, I realized I needed something from another room and left the tank with about 2 inches hanhing out over the fron edge of the stand. I am serious about there being only a few inches of water in it and nothing else, not even a lid.
When I returned to the room the glass on the end of the tank hanging out over the edge had cracked. I was lucky in that this was a small scale issue. The class on a 5.5 if not very thinck and the pressure of a bit of water and no support under the end was more than enough to break the tank. It did not take much thinking about this for me to realize how important it is to have tanks level, or as close as possible.
Our house is somewhat unusual. It is built on a conctete slab and it has one floor and no basement. Our wood floors (reinstalled in the 70s) are 1 foot square tiles of hardwood. To help insulate from the concrete they have about 1/4 inch of styrofoam attached to the bottom. It helps insulate. However, over time, between the wood and styro, the tiles can compress if there is enough weight on them. I use a lot of stands which only support the tank perimeter and the stand has no top, just the frame. This puts a lot of weight on the legs and over time it will cause a stand to go out of level slightly.
This makes it essential that all the tanks start out level. The amount of settling only changes the level by about 1/8, or a tad more, of an inch which is not an issue. The way I know it is out of level by this amount is, when I refill the tank, the surface of the water does not reach the top frame all at the same time. I will see a ting gap briefly during refill as the level begibs to reach the frame.
To get any new stand and tank set-up level at the outset I use a long level and a set of wooden shims. I check the level when the stand is put in place. Then I put the empty tank (or tanks) on the stand and recheck. But I do not do much to level things yet. I then begin to add water (to the bigger tank if there are two and they are different sizes). I fill a bit at a time checking the level as I go. This practice will cause the tank to settle into place before it is filled. I can then use my shims to correct any leveling issues.
Very few of mu tanks are actually perfectly level. However, none of them are out of level by more than a very small amount. Some start out 100% level but over the years things settle. But they never do so to the point of my needing to take remedial action. At best, I may have to whack a shim somewhere to do a minor re-balance. I just wait to do so when doing a water change. I do the shim tapping before I refill.
Over the years I have had only 4 tanks leak, The 5.5 above, my original 45 gal. went after about 10 years when it leaked at a joint about 4 inches below the top. I had my in-wall 75, which I got used, leak from the lower right corner. I moved the contents to the 150 waiting to be set up and then got a brand ne 75 to replace it. I do not reseal leaking tanks, I replace them. The 4th tak leaked when I bought it- a 20L. Back then they were so cheap (Petco still did it's buck a gallon sales) that I just replaced it.
The moral of the story is that some tanks will just give out but others we can help along this route by setting them uo out of level by more than a very small amount. One reason is the trend ti using thinner glass over the years. Thicker glass has a greater surface area to be siliconed when they are assembled. The thicker glass also helped to reduce potential bowing. Yes glass will bend some before it breaks. The first time I looked across the front glass of my 45 from the side and noticed that it bowed out slightly was the first time I understood this was possible.
Do your best to keep your tanks as level as you can. Not doing so can be inviting trouble.
, that's why I asked 
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