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 $28.33   9.734%   $31.0876   $31.09   $31.10   $31.10  
  Purchase #2 $28.88   4.207%   $30.095   $30.10   $30.10   $30.10  
  Purchase #3 $136.69   9.401%   $149.5402   $149.54   $149.55   $149.50  
  Purchase #4 $190.28   6.687%   $203.004   $203.00   $203.00   $203.00  
  Purchase #5 $150.46   8.850%   $163.7757   $163.78   $163.80   $163.80  
  Purchase #6 $123.63   8.504%   $134.1435   $134.14   $134.15   $134.10  
  Purchase #7 $219.46   10.631%   $242.7908   $242.79   $242.80   $242.80  
  7 purchase totals = $877.73       $954.4368   $954.44   $954.50   $954.40