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 $116.69   12.395%   $131.1537   $131.15   $131.15   $131.20  
  Purchase #2 $243.27   4.256%   $253.6236   $253.62   $253.60   $253.60  
  Purchase #3 $45.55   8.465%   $49.4058   $49.41   $49.40   $49.40  
  Purchase #4 $82.56   6.703%   $88.094   $88.09   $88.10   $88.10  
  Purchase #5 $69.96   10.290%   $77.1589   $77.16   $77.15   $77.20  
  Purchase #6 $245.55   9.592%   $269.1032   $269.10   $269.10   $269.10  
  Purchase #7 $24.84   13.289%   $28.141   $28.14   $28.15   $28.10  
  7 purchase totals = $828.42       $896.6802   $896.67   $896.65   $896.70