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 $237.83   13.496%   $269.9275   $269.93   $269.95   $269.90  
  Purchase #2 $58.69   6.247%   $62.3564   $62.36   $62.35   $62.40  
  Purchase #3 $167.60   7.649%   $180.4197   $180.42   $180.40   $180.40  
  Purchase #4 $132.52   12.170%   $148.6477   $148.65   $148.65   $148.60  
  Purchase #5 $227.40   13.265%   $257.5646   $257.56   $257.55   $257.60  
  Purchase #6 $112.91   11.430%   $125.8156   $125.82   $125.80   $125.80  
  Purchase #7 $5.88   12.851%   $6.6356   $6.64   $6.65   $6.60  
  7 purchase totals = $942.83       $1,051.3671   $1,051.38   $1,051.35   $1,051.30