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 $227.44   13.394%   $257.9033   $257.90   $257.90   $257.90  
  Purchase #2 $219.61   6.181%   $233.1841   $233.18   $233.20   $233.20  
  Purchase #3 $81.48   11.758%   $91.0604   $91.06   $91.05   $91.10  
  Purchase #4 $58.65   8.254%   $63.491   $63.49   $63.50   $63.50  
  Purchase #5 $237.28   5.688%   $250.7765   $250.78   $250.80   $250.80  
  Purchase #6 $67.42   7.445%   $72.4394   $72.44   $72.45   $72.40  
  Purchase #7 $40.42   8.332%   $43.7878   $43.79   $43.80   $43.80  
  7 purchase totals = $932.30       $1,012.6425   $1,012.64   $1,012.70   $1,012.70