Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $12.88   3.754%   $13.3635   $13.36   $13.35   $13.40  
  Purchase #2 $160.56   4.850%   $168.3472   $168.35   $168.35   $168.30  
  Purchase #3 $102.58   7.549%   $110.3238   $110.32   $110.30   $110.30  
  Purchase #4 $108.36   9.479%   $118.6314   $118.63   $118.65   $118.60  
  Purchase #5 $209.29   9.519%   $229.2123   $229.21   $229.20   $229.20  
  Purchase #6 $160.63   12.234%   $180.2815   $180.28   $180.30   $180.30  
  Purchase #7 $103.54   4.325%   $108.0181   $108.02   $108.00   $108.00  
  7 purchase totals = $857.84       $928.1778   $928.17   $928.15   $928.10