WESTCHESTER ASKS RISE IN SALES TAX
Westchester County Exec Alfred B DelBello says he plans to ask Bd of Legislators to raise sales taxes in county to at least 7%; believes he has bd's support; says increase will be higher if NYS sales tax is raised; says municipalities in county, which would share in revenues, also need funds; DelBello and his aides decline to comment on new budget, but it is expected to eliminate hundreds of jobs, many of them now vacant (M)