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 $248.52   12.565%   $279.7465   $279.75   $279.75   $279.70  
  Purchase #2 $179.43   13.526%   $203.6997   $203.70   $203.70   $203.70  
  Purchase #3 $42.30   6.542%   $45.0673   $45.07   $45.05   $45.10  
  Purchase #4 $183.52   8.223%   $198.6108   $198.61   $198.60   $198.60  
  Purchase #5 $61.56   4.755%   $64.4872   $64.49   $64.50   $64.50  
  Purchase #6 $21.87   11.118%   $24.3015   $24.30   $24.30   $24.30  
  Purchase #7 $194.89   11.531%   $217.3628   $217.36   $217.35   $217.40  
  7 purchase totals = $932.09       $1,033.2758   $1,033.28   $1,033.25   $1,033.30