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 $241.57   10.213%   $266.2415   $266.24   $266.25   $266.20  
  Purchase #2 $18.73   9.577%   $20.5238   $20.52   $20.50   $20.50  
  Purchase #3 $56.93   7.501%   $61.2003   $61.20   $61.20   $61.20  
  Purchase #4 $142.76   11.830%   $159.6485   $159.65   $159.65   $159.60  
  Purchase #5 $102.79   10.401%   $113.4812   $113.48   $113.50   $113.50  
  Purchase #6 $201.81   11.853%   $225.7305   $225.73   $225.75   $225.70  
  Purchase #7 $118.43   6.171%   $125.7383   $125.74   $125.75   $125.70  
  7 purchase totals = $883.02       $972.5641   $972.56   $972.60   $972.40