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 $242.39   5.579%   $255.9129   $255.91   $255.90   $255.90  
  Purchase #2 $31.57   10.195%   $34.7886   $34.79   $34.80   $34.80  
  Purchase #3 $182.56   7.203%   $195.7098   $195.71   $195.70   $195.70  
  Purchase #4 $121.48   13.311%   $137.6502   $137.65   $137.65   $137.70  
  Purchase #5 $121.82   11.387%   $135.6916   $135.69   $135.70   $135.70  
  Purchase #6 $192.83   11.161%   $214.3518   $214.35   $214.35   $214.40  
  Purchase #7 $45.70   10.740%   $50.6082   $50.61   $50.60   $50.60  
  7 purchase totals = $938.35       $1,024.7131   $1,024.71   $1,024.70   $1,024.80