I worked for a ministry last year. Sometimes I was on the ministry grounds 6 days in a row, 24 hours a day. Granted, some of that was sleep, but when I was awake, I was working, helping with the missionary teams and child day-camps. Sometimes I drove back and forth every day. The drive from my apartment to the ministry grounds was over an hour, one way. There was nothing in between, so there was no way for me to live closer. My round trip, for one day, was 3 hours at times. I slept in my office sometimes, on a blow up mattress. It was a phsyically, emotionally, spiritually, and mentally demanding job. I was spending hundreds in gas per month. I didn't have any leftover money to spend on anything else besides gas, food, rent, and utilities. In fact, I had to start using my credit card for stuff because I had no money for it.
And then the president of the ministry whom I worked for had the guts to say during a staff meeting, "How come none of my staff, no one, tithes to this ministry that they work for?"
I got kinda angry. Not only did I literally not have any money to tithe, but I figured I was tithing with my life, my time, my skills. If you have the money, I say go for it. I have a job now and I tithe. Give with a cheerful heart. Spare what you can to give back. But sometimes, you just can't, and sometimes, your time that you give and your heart are just as valuable.
Perhaps I'm wrong in my thinking...but that experience showed me that some people have a very strict idea on tithing. One that isn't necessarily...I don't know. Always realistic, I suppose.