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 $242.75   13.756%   $276.1427   $276.14   $276.15   $276.10  
  Purchase #2 $212.83   6.268%   $226.1702   $226.17   $226.15   $226.20  
  Purchase #3 $99.89   7.670%   $107.5516   $107.55   $107.55   $107.60  
  Purchase #4 $25.56   3.175%   $26.3715   $26.37   $26.35   $26.40  
  Purchase #5 $163.79   4.626%   $171.3669   $171.37   $171.35   $171.40  
  Purchase #6 $199.57   13.129%   $225.7715   $225.77   $225.75   $225.80  
  Purchase #7 $51.38   13.280%   $58.2033   $58.20   $58.20   $58.20  
  7 purchase totals = $995.77       $1,091.5777   $1,091.57   $1,091.50   $1,091.70