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 $36.78   7.544%   $39.5547   $39.55   $39.55   $39.60  
  Purchase #2 $40.47   10.794%   $44.8383   $44.84   $44.85   $44.80  
  Purchase #3 $164.82   11.165%   $183.2222   $183.22   $183.20   $183.20  
  Purchase #4 $38.39   4.856%   $40.2542   $40.25   $40.25   $40.30  
  Purchase #5 $104.84   3.726%   $108.7463   $108.75   $108.75   $108.70  
  Purchase #6 $81.51   11.829%   $91.1518   $91.15   $91.15   $91.20  
  Purchase #7 $130.70   6.685%   $139.4373   $139.44   $139.45   $139.40  
  7 purchase totals = $597.51       $647.2048   $647.20   $647.20   $647.20