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 $136.99   12.532%   $154.1576   $154.16   $154.15   $154.20  
  Purchase #2 $59.33   11.415%   $66.1025   $66.10   $66.10   $66.10  
  Purchase #3 $112.49   8.721%   $122.3003   $122.30   $122.30   $122.30  
  Purchase #4 $234.42   12.288%   $263.2255   $263.23   $263.25   $263.20  
  Purchase #5 $114.90   8.473%   $124.6355   $124.64   $124.65   $124.60  
  Purchase #6 $8.51   11.645%   $9.501   $9.50   $9.50   $9.50  
  Purchase #7 $232.96   10.574%   $257.5932   $257.59   $257.60   $257.60  
  7 purchase totals = $899.60       $997.5156   $997.52   $997.55   $997.50