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 $27.66   4.405%   $28.8784   $28.88   $28.90   $28.90  
  Purchase #2 $5.79   12.597%   $6.5194   $6.52   $6.50   $6.50  
  Purchase #3 $47.70   11.818%   $53.3372   $53.34   $53.35   $53.30  
  Purchase #4 $174.84   12.461%   $196.6268   $196.63   $196.65   $196.60  
  Purchase #5 $234.47   11.461%   $261.3426   $261.34   $261.35   $261.30  
  Purchase #6 $6.80   6.645%   $7.2519   $7.25   $7.25   $7.30  
  Purchase #7 $191.31   7.648%   $205.9414   $205.94   $205.95   $205.90  
  7 purchase totals = $688.57       $759.8977   $759.90   $759.95   $759.80