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 $211.83   13.273%   $239.9462   $239.95   $239.95   $239.90  
  Purchase #2 $180.43   6.313%   $191.8205   $191.82   $191.80   $191.80  
  Purchase #3 $129.71   13.842%   $147.6645   $147.66   $147.65   $147.70  
  Purchase #4 $87.32   4.220%   $91.0049   $91.00   $91.00   $91.00  
  Purchase #5 $222.55   12.361%   $250.0594   $250.06   $250.05   $250.10  
  Purchase #6 $230.39   7.407%   $247.455   $247.46   $247.45   $247.50  
  Purchase #7 $181.41   8.311%   $196.487   $196.49   $196.50   $196.50  
  7 purchase totals = $1,243.64       $1,364.4375   $1,364.44   $1,364.40   $1,364.50