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 $20.31   5.791%   $21.4862   $21.49   $21.50   $21.50  
  Purchase #2 $98.95   5.143%   $104.039   $104.04   $104.05   $104.00  
  Purchase #3 $141.82   8.845%   $154.364   $154.36   $154.35   $154.40  
  Purchase #4 $32.65   5.356%   $34.3987   $34.40   $34.40   $34.40  
  Purchase #5 $130.65   4.323%   $136.298   $136.30   $136.30   $136.30  
  Purchase #6 $144.81   11.314%   $161.1938   $161.19   $161.20   $161.20  
  Purchase #7 $92.27   7.434%   $99.1294   $99.13   $99.15   $99.10  
  7 purchase totals = $661.46       $710.9091   $710.91   $710.95   $710.90