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 $54.26   6.726%   $57.9095   $57.91   $57.90   $57.90  
  Purchase #2 $170.61   12.480%   $191.9021   $191.90   $191.90   $191.90  
  Purchase #3 $133.46   13.588%   $151.5945   $151.59   $151.60   $151.60  
  Purchase #4 $64.64   6.167%   $68.6263   $68.63   $68.65   $68.60  
  Purchase #5 $7.52   5.845%   $7.9595   $7.96   $7.95   $8.00  
  Purchase #6 $96.34   10.446%   $106.4037   $106.40   $106.40   $106.40  
  Purchase #7 $128.84   3.162%   $132.9139   $132.91   $132.90   $132.90  
  7 purchase totals = $655.67       $717.3095   $717.30   $717.30   $717.30