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 $17.33   4.338%   $18.0818   $18.08   $18.10   $18.10  
  Purchase #2 $31.97   9.670%   $35.0615   $35.06   $35.05   $35.10  
  Purchase #3 $103.47   8.541%   $112.3074   $112.31   $112.30   $112.30  
  Purchase #4 $135.33   3.159%   $139.6051   $139.61   $139.60   $139.60  
  Purchase #5 $26.99   5.383%   $28.4429   $28.44   $28.45   $28.40  
  Purchase #6 $58.31   12.731%   $65.7334   $65.73   $65.75   $65.70  
  Purchase #7 $231.70   4.288%   $241.6353   $241.64   $241.65   $241.60  
  7 purchase totals = $605.10       $640.8674   $640.87   $640.90   $640.80