I am playing a Sims clan, none of whom has ever had an official job. They have all focused on writing and painting to amass their fortune.
My anecdotal experience (for what it's worth) is consistent with the guide Schipperke linked.
On a personal note, these writer/artist sims have exceed my wildest expectations re: financial viability. They are sitting on over $1,000,000 simoleans in the bank and have spent like drunken sailors. I'm seriously running out of things to buy.