Getting unsecured credit is harder for contractors, mortgages, and car loans a bit easier.
Credit underwriters are looking for stability in a job (character) - OR - a job that's in a stable industry. 2 years of a steady income in a demand field should be as good as income from a job. For example, a self-employed restaurant designer may make $20K for a month's work -- but in a difficult market segment -- he/she will have a much harder time getting credit than a contract nurse who makes $10K a month, and only works 6 mos a year.
If you are going alone and might need some reserve cash to carry you through tough spots, it's always better to get an unsecured line of credit while employed. Getting it later (when you really need it) can be difficult and/or expensive.
I was actually inquiring about investment income (cap gains, dividend and interest), not contracting income. The bank said they'd accept any income as long as it's demonstrably consistent over the last 2 years.