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 $248.49   6.843%   $265.4942   $265.49   $265.50   $265.50  
  Purchase #2 $111.87   12.312%   $125.6434   $125.64   $125.65   $125.60  
  Purchase #3 $179.30   11.588%   $200.0773   $200.08   $200.10   $200.10  
  Purchase #4 $34.34   13.727%   $39.0539   $39.05   $39.05   $39.10  
  Purchase #5 $95.41   12.308%   $107.1531   $107.15   $107.15   $107.20  
  Purchase #6 $80.25   3.799%   $83.2987   $83.30   $83.30   $83.30  
  Purchase #7 $212.54   12.400%   $238.895   $238.90   $238.90   $238.90  
  7 purchase totals = $962.20       $1,059.6156   $1,059.61   $1,059.65   $1,059.70