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 $28.96   9.737%   $31.7798   $31.78   $31.80   $31.80  
  Purchase #2 $229.27   9.255%   $250.4889   $250.49   $250.50   $250.50  
  Purchase #3 $30.49   8.602%   $33.1127   $33.11   $33.10   $33.10  
  Purchase #4 $121.87   5.240%   $128.256   $128.26   $128.25   $128.30  
  Purchase #5 $30.63   7.397%   $32.8957   $32.90   $32.90   $32.90  
  Purchase #6 $104.27   12.409%   $117.2089   $117.21   $117.20   $117.20  
  Purchase #7 $42.51   9.232%   $46.4345   $46.43   $46.45   $46.40  
  7 purchase totals = $588.00       $640.1765   $640.18   $640.20   $640.20