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 $245.25   11.697%   $273.9369   $273.94   $273.95   $273.90  
  Purchase #2 $235.57   13.275%   $266.8419   $266.84   $266.85   $266.80  
  Purchase #3 $128.75   5.285%   $135.5544   $135.55   $135.55   $135.60  
  Purchase #4 $192.61   12.737%   $217.1427   $217.14   $217.15   $217.10  
  Purchase #5 $25.80   4.670%   $27.0049   $27.00   $27.00   $27.00  
  Purchase #6 $219.42   10.440%   $242.3274   $242.33   $242.35   $242.30  
  Purchase #7 $57.54   6.249%   $61.1357   $61.14   $61.15   $61.10  
  7 purchase totals = $1,104.94       $1,223.9439   $1,223.94   $1,224.00   $1,223.80