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 $57.84   13.250%   $65.5038   $65.50   $65.50   $65.50  
  Purchase #2 $123.57   9.506%   $135.3166   $135.32   $135.30   $135.30  
  Purchase #3 $13.71   11.431%   $15.2772   $15.28   $15.30   $15.30  
  Purchase #4 $53.76   10.724%   $59.5252   $59.53   $59.55   $59.50  
  Purchase #5 $5.78   6.822%   $6.1743   $6.17   $6.15   $6.20  
  Purchase #6 $226.78   6.834%   $242.2781   $242.28   $242.30   $242.30  
  Purchase #7 $101.99   10.402%   $112.599   $112.60   $112.60   $112.60  
  7 purchase totals = $583.43       $636.6742   $636.68   $636.70   $636.70