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 $248.79   13.266%   $281.7945   $281.79   $281.80   $281.80  
  Purchase #2 $175.55   4.599%   $183.6235   $183.62   $183.60   $183.60  
  Purchase #3 $166.46   6.418%   $177.1434   $177.14   $177.15   $177.10  
  Purchase #4 $96.94   7.404%   $104.1174   $104.12   $104.10   $104.10  
  Purchase #5 $109.95   7.544%   $118.2446   $118.24   $118.25   $118.20  
  Purchase #6 $217.25   5.615%   $229.4486   $229.45   $229.45   $229.40  
  Purchase #7 $214.52   9.198%   $234.2515   $234.25   $234.25   $234.30  
  7 purchase totals = $1,229.46       $1,328.6235   $1,328.61   $1,328.60   $1,328.50