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 $236.84   5.565%   $250.0201   $250.02   $250.00   $250.00  
  Purchase #2 $122.85   10.552%   $135.8131   $135.81   $135.80   $135.80  
  Purchase #3 $25.85   11.457%   $28.8116   $28.81   $28.80   $28.80  
  Purchase #4 $90.81   4.329%   $94.7412   $94.74   $94.75   $94.70  
  Purchase #5 $107.51   13.157%   $121.6551   $121.66   $121.65   $121.70  
  Purchase #6 $243.51   6.690%   $259.8008   $259.80   $259.80   $259.80  
  Purchase #7 $60.43   7.643%   $65.0487   $65.05   $65.05   $65.00  
  7 purchase totals = $887.80       $955.8906   $955.89   $955.85   $955.80