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 $238.69   11.723%   $266.6716   $266.67   $266.65   $266.70  
  Purchase #2 $35.66   6.609%   $38.0168   $38.02   $38.00   $38.00  
  Purchase #3 $150.60   12.479%   $169.3934   $169.39   $169.40   $169.40  
  Purchase #4 $122.29   11.435%   $136.2739   $136.27   $136.25   $136.30  
  Purchase #5 $239.48   7.391%   $257.18   $257.18   $257.20   $257.20  
  Purchase #6 $25.64   4.104%   $26.6923   $26.69   $26.70   $26.70  
  Purchase #7 $24.89   10.293%   $27.4519   $27.45   $27.45   $27.50  
  7 purchase totals = $837.25       $921.6799   $921.67   $921.65   $921.80