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 $28.78   5.685%   $30.4161   $30.42   $30.40   $30.40  
  Purchase #2 $229.52   12.385%   $257.9461   $257.95   $257.95   $257.90  
  Purchase #3 $186.94   6.659%   $199.3883   $199.39   $199.40   $199.40  
  Purchase #4 $171.87   6.145%   $182.4314   $182.43   $182.45   $182.40  
  Purchase #5 $115.29   5.620%   $121.7693   $121.77   $121.75   $121.80  
  Purchase #6 $122.67   4.604%   $128.3177   $128.32   $128.30   $128.30  
  Purchase #7 $34.56   13.651%   $39.2778   $39.28   $39.30   $39.30  
  7 purchase totals = $889.63       $959.5467   $959.56   $959.55   $959.50