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 $130.80   6.178%   $138.8808   $138.88   $138.90   $138.90  
  Purchase #2 $70.85   9.821%   $77.8082   $77.81   $77.80   $77.80  
  Purchase #3 $237.77   5.494%   $250.8331   $250.83   $250.85   $250.80  
  Purchase #4 $149.71   5.252%   $157.5728   $157.57   $157.55   $157.60  
  Purchase #5 $51.81   4.676%   $54.2326   $54.23   $54.25   $54.20  
  Purchase #6 $11.84   9.428%   $12.9563   $12.96   $12.95   $13.00  
  Purchase #7 $159.92   6.699%   $170.633   $170.63   $170.65   $170.60  
  7 purchase totals = $812.70       $862.9168   $862.91   $862.95   $862.90