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 $212.38   13.544%   $241.1447   $241.14   $241.15   $241.10  
  Purchase #2 $173.99   11.583%   $194.1433   $194.14   $194.15   $194.10  
  Purchase #3 $146.46   11.525%   $163.3395   $163.34   $163.35   $163.30  
  Purchase #4 $27.52   7.474%   $29.5768   $29.58   $29.60   $29.60  
  Purchase #5 $23.31   5.693%   $24.637   $24.64   $24.65   $24.60  
  Purchase #6 $137.62   4.134%   $143.3092   $143.31   $143.30   $143.30  
  Purchase #7 $39.72   8.730%   $43.1876   $43.19   $43.20   $43.20  
  7 purchase totals = $761.00       $839.3381   $839.34   $839.40   $839.20