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 $37.34   5.503%   $39.3948   $39.39   $39.40   $39.40  
  Purchase #2 $39.48   7.337%   $42.3766   $42.38   $42.40   $42.40  
  Purchase #3 $35.72   5.701%   $37.7564   $37.76   $37.75   $37.80  
  Purchase #4 $191.73   13.719%   $218.0334   $218.03   $218.05   $218.00  
  Purchase #5 $209.71   13.160%   $237.3078   $237.31   $237.30   $237.30  
  Purchase #6 $122.59   10.756%   $135.7758   $135.78   $135.80   $135.80  
  Purchase #7 $187.97   12.372%   $211.2256   $211.23   $211.25   $211.20  
  7 purchase totals = $824.54       $921.8704   $921.88   $921.95   $921.90