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 $240.27   13.728%   $273.2543   $273.25   $273.25   $273.30  
  Purchase #2 $171.69   9.866%   $188.6289   $188.63   $188.65   $188.60  
  Purchase #3 $211.96   7.701%   $228.283   $228.28   $228.30   $228.30  
  Purchase #4 $163.90   9.629%   $179.6819   $179.68   $179.70   $179.70  
  Purchase #5 $94.99   13.460%   $107.7757   $107.78   $107.80   $107.80  
  Purchase #6 $212.46   5.469%   $224.0794   $224.08   $224.10   $224.10  
  Purchase #7 $58.32   6.479%   $62.0986   $62.10   $62.10   $62.10  
  7 purchase totals = $1,153.59       $1,263.8018   $1,263.80   $1,263.90   $1,263.90