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 $35.37   12.710%   $39.8655   $39.87   $39.85   $39.90  
  Purchase #2 $39.91   8.808%   $43.4253   $43.43   $43.45   $43.40  
  Purchase #3 $20.76   11.194%   $23.0839   $23.08   $23.10   $23.10  
  Purchase #4 $106.57   10.423%   $117.6778   $117.68   $117.70   $117.70  
  Purchase #5 $225.59   13.663%   $256.4124   $256.41   $256.40   $256.40  
  Purchase #6 $216.68   8.485%   $235.0653   $235.07   $235.05   $235.10  
  Purchase #7 $164.86   6.356%   $175.3385   $175.34   $175.35   $175.30  
  7 purchase totals = $809.74       $890.8687   $890.88   $890.90   $890.90