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 $227.72   8.126%   $246.2245   $246.22   $246.20   $246.20  
  Purchase #2 $23.64   12.742%   $26.6522   $26.65   $26.65   $26.70  
  Purchase #3 $133.59   5.708%   $141.2153   $141.22   $141.20   $141.20  
  Purchase #4 $131.70   13.832%   $149.9167   $149.92   $149.90   $149.90  
  Purchase #5 $92.57   7.391%   $99.4118   $99.41   $99.40   $99.40  
  Purchase #6 $88.59   11.301%   $98.6016   $98.60   $98.60   $98.60  
  Purchase #7 $71.30   13.163%   $80.6852   $80.69   $80.70   $80.70  
  7 purchase totals = $769.11       $842.7073   $842.71   $842.65   $842.70