As rates creep up, borrowers with interest-only loans face sharply higher mortgage payments: Tina Gren Clarence saves $600 a month with an interest-only loan she took out about a year ago for a Petaluma home, yet already is considering heading off the financial hit when her monthly payments could jump. And Peter Shidler, whose low-payment-option loan could turn from a sweet deal into a potentially bitter one, is refinancing and still will pay $500 more monthly on his Santa Rosa home. Gren Clarence and Shidler, like two out of three Sonoma County home buyers and owners, joined the fast-growing club...