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 $21.44   5.271%   $22.5701   $22.57   $22.55   $22.60  
  Purchase #2 $20.79   13.815%   $23.6621   $23.66   $23.65   $23.70  
  Purchase #3 $61.88   13.563%   $70.2728   $70.27   $70.25   $70.30  
  Purchase #4 $158.75   11.578%   $177.1301   $177.13   $177.15   $177.10  
  Purchase #5 $128.44   3.438%   $132.8558   $132.86   $132.85   $132.90  
  Purchase #6 $10.77   13.696%   $12.2451   $12.25   $12.25   $12.20  
  Purchase #7 $181.31   4.503%   $189.4744   $189.47   $189.45   $189.50  
  7 purchase totals = $583.38       $628.2104   $628.21   $628.15   $628.30