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 $74.73   8.381%   $80.9931   $80.99   $81.00   $81.00  
  Purchase #2 $106.32   13.593%   $120.7721   $120.77   $120.75   $120.80  
  Purchase #3 $38.76   3.454%   $40.0988   $40.10   $40.10   $40.10  
  Purchase #4 $191.29   3.701%   $198.3696   $198.37   $198.35   $198.40  
  Purchase #5 $139.32   4.459%   $145.5323   $145.53   $145.55   $145.50  
  Purchase #6 $171.50   4.583%   $179.3598   $179.36   $179.35   $179.40  
  Purchase #7 $109.38   4.368%   $114.1577   $114.16   $114.15   $114.20  
  7 purchase totals = $831.30       $879.2834   $879.28   $879.25   $879.40