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 $236.60   12.739%   $266.7405   $266.74   $266.75   $266.70  
  Purchase #2 $72.47   3.394%   $74.9296   $74.93   $74.95   $74.90  
  Purchase #3 $169.62   6.427%   $180.5215   $180.52   $180.50   $180.50  
  Purchase #4 $65.79   6.310%   $69.9413   $69.94   $69.95   $69.90  
  Purchase #5 $246.82   6.446%   $262.73   $262.73   $262.75   $262.70  
  Purchase #6 $108.26   11.194%   $120.3786   $120.38   $120.40   $120.40  
  Purchase #7 $222.74   13.804%   $253.487   $253.49   $253.50   $253.50  
  7 purchase totals = $1,122.30       $1,228.7285   $1,228.73   $1,228.80   $1,228.60