People say they value customer service and fast checkout lanes when they have to go grocery shopping, but…these days, research is showing shoppers often choose their grocer based on two things, price and convenience. Result: Wal-Mart is the nation’s No. 1 grocer, even though it ranks among the least-beloved store chains. On Tuesday, Life Inc. asked readers what bothers them most about grocery stores. More than 50,000 readers responded, and although many said they don’t like dirty stores and long lines, the most common gripe was high prices. More than four in 10 respondents told Consumer Reports they had switched supermarkets because of high prices.