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 $56.35   12.676%   $63.4929   $63.49   $63.50   $63.50  
  Purchase #2 $17.47   11.668%   $19.5084   $19.51   $19.50   $19.50  
  Purchase #3 $102.44   4.294%   $106.8388   $106.84   $106.85   $106.80  
  Purchase #4 $208.99   6.240%   $222.031   $222.03   $222.05   $222.00  
  Purchase #5 $193.45   3.716%   $200.6386   $200.64   $200.65   $200.60  
  Purchase #6 $153.95   3.323%   $159.0658   $159.07   $159.05   $159.10  
  Purchase #7 $241.76   3.514%   $250.2554   $250.26   $250.25   $250.30  
  7 purchase totals = $974.41       $1,021.8309   $1,021.84   $1,021.85   $1,021.80