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 $26.58   6.235%   $28.2373   $28.24   $28.25   $28.20  
  Purchase #2 $24.81   13.500%   $28.1594   $28.16   $28.15   $28.20  
  Purchase #3 $236.59   9.711%   $259.5653   $259.57   $259.55   $259.60  
  Purchase #4 $38.34   11.353%   $42.6927   $42.69   $42.70   $42.70  
  Purchase #5 $246.56   13.476%   $279.7864   $279.79   $279.80   $279.80  
  Purchase #6 $18.33   7.599%   $19.7229   $19.72   $19.70   $19.70  
  Purchase #7 $145.80   6.145%   $154.7594   $154.76   $154.75   $154.80  
  7 purchase totals = $737.01       $812.9234   $812.93   $812.90   $813.00