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 $34.99   11.287%   $38.9393   $38.94   $38.95   $38.90  
  Purchase #2 $230.79   13.793%   $262.6229   $262.62   $262.60   $262.60  
  Purchase #3 $115.53   10.579%   $127.7519   $127.75   $127.75   $127.80  
  Purchase #4 $180.36   6.156%   $191.463   $191.46   $191.45   $191.50  
  Purchase #5 $8.81   5.427%   $9.2881   $9.29   $9.30   $9.30  
  Purchase #6 $193.41   12.705%   $217.9827   $217.98   $218.00   $218.00  
  Purchase #7 $155.29   11.747%   $173.5319   $173.53   $173.55   $173.50  
  7 purchase totals = $919.18       $1,021.5798   $1,021.57   $1,021.60   $1,021.60