Excellent point - and I'd say that the percentage should equal the age of your home, divided by 5.
1-5 Yrs Old: 1%
6-10: 2%
11-15: 3%
16-20: 4%
20+: 5%
Cause when that furnace dies in your 30 year old house, 1% probably won't cut it unless you have a $300,000 house.