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 $18.98   10.588%   $20.9896   $20.99   $21.00   $21.00  
  Purchase #2 $21.46   8.332%   $23.248   $23.25   $23.25   $23.20  
  Purchase #3 $243.60   11.274%   $271.0635   $271.06   $271.05   $271.10  
  Purchase #4 $220.62   5.552%   $232.8688   $232.87   $232.85   $232.90  
  Purchase #5 $208.79   9.455%   $228.5311   $228.53   $228.55   $228.50  
  Purchase #6 $199.86   12.475%   $224.7925   $224.79   $224.80   $224.80  
  Purchase #7 $125.84   13.708%   $143.0901   $143.09   $143.10   $143.10  
  7 purchase totals = $1,039.15       $1,144.5836   $1,144.58   $1,144.60   $1,144.60