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 $18.60   6.387%   $19.788   $19.79   $19.80   $19.80  
  Purchase #2 $20.90   7.522%   $22.4721   $22.47   $22.45   $22.50  
  Purchase #3 $115.64   9.690%   $126.8455   $126.85   $126.85   $126.80  
  Purchase #4 $32.32   9.480%   $35.3839   $35.38   $35.40   $35.40  
  Purchase #5 $88.30   3.138%   $91.0709   $91.07   $91.05   $91.10  
  Purchase #6 $174.91   8.214%   $189.2771   $189.28   $189.30   $189.30  
  Purchase #7 $143.89   3.686%   $149.1938   $149.19   $149.20   $149.20  
  7 purchase totals = $594.56       $634.0313   $634.03   $634.05   $634.10