It's kind of like asking "what's your opinion on modding?"
I play using Fortress Defense Mod. I've modded plants to take 4x longer to grow and outdoor plants are seasonal and underground aren't. I've modded steel production to get 4 steel bars per flux stone.
I use DFHack prospect all, reveal (occasionally), autodump (occasionally), cleanowned, vdig (though that's scarcely an exploit). Usually I use these things when I think the dwarves should be able to do these things (for example using autodump to recover an item which has fallen into a pond, which a dwarf should be able to fish out with a long pole. And I attribute the prospect/reveal stuff with dwarves being good prospectors and/or having a supernatural ability to sense metals).
I refuse to buy or use training axes for woodcutting. On embark I don't buy single units of food etc to get freebie barrels/bags. But I do buy metal ores rather than bars to get bars at about 1/16th the cost, and I buy raw materials rather than finished goods - basically the prices just aren't balanced at all although the intent probably is that metal bars are the cost you "should" be looking at and the raw materials should be maybe half as expensive at most, but not 1/16th. Gold nuggets basically "should" cost 4x as much as gold bars and native gold blocks.
Basically how I see it, is a wooden axe shouldn't be able to cut down a tree. You shouldn't be able to buy a wombat heart and a barrel, for less than buying a barrel without a wombat heart. You should be able to save money by buying raw materials and processing them. So if it's a matter of the balance being off, I exploit heartily. But if it's a matter of defiance of common sense, I usually refrain.