Adverb placement is generally interchangeable, but there is a subtle difference in meaning in some cases due to the placement altering the emphasis in the sentence. Take for example: 

I only love you.

I love only you.

The first can be taken in a way to suggest that the speaker does nothing in life but love the person they're speaking to. The second is more definitively an admission of the exclusive nature of that love. 

But I'm being super pedantic and I apologize. :)

The only serious rule I can think of regarding adverb placement is to not insert the adverb between a transitive verb and its object. e.g.,

She plays the piano beautifully.

Or

She beautifully plays the piano. (Though this sounds less natural to me)

Not

She plays beautifully the piano.

Either way is correct

'I mainly depend on you' and 'I trade mainly in stocks' are also correct.