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 $170.58   4.872%   $178.8907   $178.89   $178.90   $178.90  
  Purchase #2 $30.86   5.781%   $32.644   $32.64   $32.65   $32.60  
  Purchase #3 $201.38   10.825%   $223.1794   $223.18   $223.20   $223.20  
  Purchase #4 $10.98   10.277%   $12.1084   $12.11   $12.10   $12.10  
  Purchase #5 $10.27   7.824%   $11.0735   $11.07   $11.05   $11.10  
  Purchase #6 $8.72   3.500%   $9.0252   $9.03   $9.05   $9.00  
  Purchase #7 $68.81   9.785%   $75.5431   $75.54   $75.55   $75.50  
  7 purchase totals = $501.60       $542.4643   $542.46   $542.50   $542.40