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 $17.46   3.768%   $18.1179   $18.12   $18.10   $18.10  
  Purchase #2 $15.64   12.641%   $17.6171   $17.62   $17.60   $17.60  
  Purchase #3 $80.76   6.420%   $85.9448   $85.94   $85.95   $85.90  
  Purchase #4 $243.76   3.335%   $251.8894   $251.89   $251.90   $251.90  
  Purchase #5 $7.56   11.571%   $8.4348   $8.43   $8.45   $8.40  
  Purchase #6 $35.30   11.795%   $39.4636   $39.46   $39.45   $39.50  
  Purchase #7 $160.89   12.715%   $181.3472   $181.35   $181.35   $181.30  
  7 purchase totals = $561.37       $602.8148   $602.81   $602.80   $602.70