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 $179.95   4.130%   $187.3819   $187.38   $187.40   $187.40  
  Purchase #2 $123.49   5.184%   $129.8917   $129.89   $129.90   $129.90  
  Purchase #3 $122.89   7.322%   $131.888   $131.89   $131.90   $131.90  
  Purchase #4 $210.42   9.818%   $231.079   $231.08   $231.10   $231.10  
  Purchase #5 $78.33   12.832%   $88.3813   $88.38   $88.40   $88.40  
  Purchase #6 $39.64   12.787%   $44.7088   $44.71   $44.70   $44.70  
  Purchase #7 $110.82   5.270%   $116.6602   $116.66   $116.65   $116.70  
  7 purchase totals = $865.54       $929.9909   $929.99   $930.05   $930.10