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 $146.45   12.642%   $164.9642   $164.96   $164.95   $165.00  
  Purchase #2 $97.37   7.457%   $104.6309   $104.63   $104.65   $104.60  
  Purchase #3 $163.48   7.618%   $175.9339   $175.93   $175.95   $175.90  
  Purchase #4 $79.94   8.408%   $86.6614   $86.66   $86.65   $86.70  
  Purchase #5 $67.94   4.604%   $71.068   $71.07   $71.05   $71.10  
  Purchase #6 $142.26   6.665%   $151.7416   $151.74   $151.75   $151.70  
  Purchase #7 $222.97   5.369%   $234.9413   $234.94   $234.95   $234.90  
  7 purchase totals = $920.41       $989.9413   $989.93   $989.95   $989.90