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 $120.97   11.151%   $134.4594   $134.46   $134.45   $134.50  
  Purchase #2 $138.35   4.597%   $144.7099   $144.71   $144.70   $144.70  
  Purchase #3 $152.62   11.102%   $169.5639   $169.56   $169.55   $169.60  
  Purchase #4 $124.57   9.827%   $136.8115   $136.81   $136.80   $136.80  
  Purchase #5 $59.52   12.730%   $67.0969   $67.10   $67.10   $67.10  
  Purchase #6 $8.58   13.578%   $9.745   $9.75   $9.75   $9.70  
  Purchase #7 $162.77   10.829%   $180.3964   $180.40   $180.40   $180.40  
  7 purchase totals = $767.38       $842.7830   $842.79   $842.75   $842.80