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 $20.35   4.438%   $21.2531   $21.25   $21.25   $21.30  
  Purchase #2 $20.34   7.152%   $21.7947   $21.79   $21.80   $21.80  
  Purchase #3 $224.99   6.272%   $239.1014   $239.10   $239.10   $239.10  
  Purchase #4 $249.30   8.549%   $270.6127   $270.61   $270.60   $270.60  
  Purchase #5 $193.33   9.295%   $211.3   $211.30   $211.30   $211.30  
  Purchase #6 $226.61   3.778%   $235.1713   $235.17   $235.15   $235.20  
  Purchase #7 $122.64   6.699%   $130.8557   $130.86   $130.85   $130.90  
  7 purchase totals = $1,057.56       $1,130.0889   $1,130.08   $1,130.05   $1,130.20