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 $244.28   12.407%   $274.5878   $274.59   $274.60   $274.60  
  Purchase #2 $27.29   4.716%   $28.577   $28.58   $28.60   $28.60  
  Purchase #3 $122.60   10.116%   $135.0022   $135.00   $135.00   $135.00  
  Purchase #4 $69.99   8.124%   $75.676   $75.68   $75.70   $75.70  
  Purchase #5 $67.34   11.549%   $75.1171   $75.12   $75.10   $75.10  
  Purchase #6 $109.78   8.809%   $119.4505   $119.45   $119.45   $119.50  
  Purchase #7 $48.93   4.733%   $51.2459   $51.25   $51.25   $51.20  
  7 purchase totals = $690.21       $759.6565   $759.67   $759.70   $759.70