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 $206.37   13.782%   $234.8119   $234.81   $234.80   $234.80  
  Purchase #2 $21.79   12.458%   $24.5046   $24.50   $24.50   $24.50  
  Purchase #3 $131.36   4.228%   $136.9139   $136.91   $136.90   $136.90  
  Purchase #4 $157.89   7.467%   $169.6796   $169.68   $169.70   $169.70  
  Purchase #5 $53.75   12.671%   $60.5607   $60.56   $60.55   $60.60  
  Purchase #6 $159.58   6.840%   $170.4953   $170.50   $170.50   $170.50  
  Purchase #7 $153.97   4.427%   $160.7863   $160.79   $160.80   $160.80  
  7 purchase totals = $884.71       $957.7523   $957.75   $957.75   $957.80