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 $248.92   11.146%   $276.6646   $276.66   $276.65   $276.70  
  Purchase #2 $44.55   8.311%   $48.2526   $48.25   $48.25   $48.30  
  Purchase #3 $97.87   12.624%   $110.2251   $110.23   $110.25   $110.20  
  Purchase #4 $58.40   7.122%   $62.5592   $62.56   $62.55   $62.60  
  Purchase #5 $212.56   10.194%   $234.2284   $234.23   $234.25   $234.20  
  Purchase #6 $15.44   5.675%   $16.3162   $16.32   $16.30   $16.30  
  Purchase #7 $166.57   4.844%   $174.6387   $174.64   $174.65   $174.60  
  7 purchase totals = $844.31       $922.8848   $922.89   $922.90   $922.90