There's no such thing as cheating in Minecraft. Play the game any way you like. Play in creative, play in survival peaceful, play with cheats on, play with cheats off. Play with mods, play with resource packs. Whatever you enjoy doing.
Here's the way I play: I use mapping tools when I want to find the nearest jungle, or mushroom island, or whatever.
I use the seed of my survival world to make a duplicate world in creative so I can fly around and check out the world.
Sometimes, when I really, really need to, I use "Open to LAN" to turn cheats on in my survival world.
When I want beautiful buildings I use one of several resource packs.
The only time I don't use any aids or cheats of any kind is when I make a hardcore world, totally at random. When I play hardcore, I play hardcore! But then I don't really expect to survive my hardcore quickie games for more than a few real-life days.
Hello. I want to know if using third-party tools like admist or minealtas is cheating? I use them on my worlds, and I want to know. Thanks.
It's your world, it's up to you to decide how to play!
Some people like to use creative mode, peaceful...no mobs, and infinite resources.
Others wouldn't dream of using AMIDST, because they like the challenge.
It's not all that hard to find slime-chunks without AMIDST - it just takes a bit of time digging out a large area, and waiting to see which bits they spawn in. Because of all the digging, it's best after getting a beacon, really.
1/10 chunks is a slime chunk. If you use F3 to dig out up to the boundray of a chunk - at least 3 wide, preferably more, and preferably 3 high - light it up, then leave the wall... and dig another chunk....I usually put a fence-gate in between them, so I know for sure which one they spawned in. Then I just wait a while (more than 24 blocks away, but less than 128 away) - and listen out for squishys.
Or, sure - if you feel OK about it - just use AMIDST.
That's one of the great things about Minecraft... everyone can play it their own way.
IMO, if you have to ask if something is cheating, or feel guilty about doing it then it probably is - to you (in singleplayer, as multiplayer has its own rules).
Personally, I don't want to find things like biomes and structures with external utilities; I've even modded the game to ensure that a utility that I use to map my world (generated chunks) only shows what I've explored underground (I removed naturally generated torches from mineshafts and strongholds; the utility has an underground rendering mode that only renders caves if they have torches so this prevents it from showing them, as in this example, note the railway extending to the upper-right, indicating that chunks were generated there, while caves only render as I explore them). I even reduced the view distance (always 10 chunks in 1.6.4) to 8 chunks so I do not generate more chunks than an in-game map can display (this is a major reason why I still use Normal render distance despite having a computer that can easily handle Far).
However, I have used AMIDST to find a suitable seed before (as in a large landmass around spawn) but it is practically useless in finding biomes or structures due to my mods, and I did not use it at all for my most recent world (the chances of an ocean spawn aren't that high anyway, particularly with my recent mods, and you can always use another seed); likewise, I've reused seeds but it is the underground, which was altered, that matters the most. In a couple other worlds I used MCEdit to move a base over from a previous world but those worlds were mainly "test" worlds while experimenting with various modifications to the underground.