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 $229.63   13.252%   $260.0606   $260.06   $260.05   $260.10  
  Purchase #2 $56.50   11.219%   $62.8387   $62.84   $62.85   $62.80  
  Purchase #3 $110.93   3.342%   $114.6373   $114.64   $114.65   $114.60  
  Purchase #4 $116.63   9.193%   $127.3518   $127.35   $127.35   $127.40  
  Purchase #5 $230.62   8.395%   $249.9805   $249.98   $250.00   $250.00  
  Purchase #6 $97.25   9.207%   $106.2038   $106.20   $106.20   $106.20  
  Purchase #7 $7.65   10.697%   $8.4683   $8.47   $8.45   $8.50  
  7 purchase totals = $849.21       $929.5410   $929.54   $929.55   $929.60