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 $244.64   11.866%   $273.669   $273.67   $273.65   $273.70  
  Purchase #2 $66.85   4.464%   $69.8342   $69.83   $69.85   $69.80  
  Purchase #3 $139.42   11.807%   $155.8813   $155.88   $155.90   $155.90  
  Purchase #4 $127.74   12.222%   $143.3524   $143.35   $143.35   $143.40  
  Purchase #5 $249.54   10.795%   $276.4778   $276.48   $276.50   $276.50  
  Purchase #6 $239.41   4.346%   $249.8148   $249.81   $249.80   $249.80  
  Purchase #7 $136.79   5.413%   $144.1944   $144.19   $144.20   $144.20  
  7 purchase totals = $1,204.39       $1,313.2239   $1,313.21   $1,313.25   $1,313.30