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 $213.54   12.118%   $239.4168   $239.42   $239.40   $239.40  
  Purchase #2 $110.87   10.527%   $122.5413   $122.54   $122.55   $122.50  
  Purchase #3 $11.34   4.177%   $11.8137   $11.81   $11.80   $11.80  
  Purchase #4 $56.78   7.754%   $61.1827   $61.18   $61.20   $61.20  
  Purchase #5 $95.87   11.504%   $106.8989   $106.90   $106.90   $106.90  
  Purchase #6 $110.72   12.658%   $124.7349   $124.73   $124.75   $124.70  
  Purchase #7 $240.71   8.297%   $260.6817   $260.68   $260.70   $260.70  
  7 purchase totals = $839.83       $927.2700   $927.26   $927.30   $927.20