My personal experience:
No, you don’t show your income.
You choose net or gross, but my church loved to preach that your blessing would be greater if your tithes were greater. They also kind of discouraged getting the tax break for the donations, though never in so many words.
Tithes were the beginning, not the end, of your giving. If you were going through hard times, you were encouraged to give more, not less so God could increase your blessing.
The tithes were, essentially, the pastor’s paycheck. He paid for the church’s bills with the pass-the-basket offerings and the Faith Promise monthly offering and the building fund.
I was on the ministry staff at one point, and had access to the list of “voting members” – that is, people who tithed at least once a month.
We were a small church, but at one point my pastor was pulling something like 15 times the minimum wage for a full time job. Enough to build a new house, buy a boat and new cars for himself, his wife, and three daughters, and keep them all in bespoke church clothes. This is all while members of his congregation lived in government housing and couldn’t afford Wal-Mart clothes for their kids.
And no, there was no homeless ministry or food pantry or anything else. They did go to Thailand once a year, to visit their tailor…