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 $38.27   12.150%   $42.9198   $42.92   $42.90   $42.90  
  Purchase #2 $130.50   13.393%   $147.9779   $147.98   $148.00   $148.00  
  Purchase #3 $110.56   3.575%   $114.5125   $114.51   $114.50   $114.50  
  Purchase #4 $103.63   6.315%   $110.1742   $110.17   $110.15   $110.20  
  Purchase #5 $75.58   9.369%   $82.6611   $82.66   $82.65   $82.70  
  Purchase #6 $186.97   8.410%   $202.6942   $202.69   $202.70   $202.70  
  Purchase #7 $100.61   11.867%   $112.5494   $112.55   $112.55   $112.50  
  7 purchase totals = $746.12       $813.4891   $813.48   $813.45   $813.50