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 $125.93   12.212%   $141.3086   $141.31   $141.30   $141.30  
  Purchase #2 $234.62   10.194%   $258.5372   $258.54   $258.55   $258.50  
  Purchase #3 $80.81   8.529%   $87.7023   $87.70   $87.70   $87.70  
  Purchase #4 $219.90   11.612%   $245.4348   $245.43   $245.45   $245.40  
  Purchase #5 $33.41   6.157%   $35.4671   $35.47   $35.45   $35.50  
  Purchase #6 $17.72   9.529%   $19.4085   $19.41   $19.40   $19.40  
  Purchase #7 $13.62   3.794%   $14.1367   $14.14   $14.15   $14.10  
  7 purchase totals = $726.01       $801.9952   $802.00   $802.00   $801.90