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 $29.37   3.527%   $30.4059   $30.41   $30.40   $30.40  
  Purchase #2 $54.89   6.462%   $58.437   $58.44   $58.45   $58.40  
  Purchase #3 $158.96   13.388%   $180.2416   $180.24   $180.25   $180.20  
  Purchase #4 $13.25   8.744%   $14.4086   $14.41   $14.40   $14.40  
  Purchase #5 $138.56   8.597%   $150.472   $150.47   $150.45   $150.50  
  Purchase #6 $152.51   3.436%   $157.7502   $157.75   $157.75   $157.80  
  Purchase #7 $86.81   5.678%   $91.7391   $91.74   $91.75   $91.70  
  7 purchase totals = $634.35       $683.4544   $683.46   $683.45   $683.40