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 $248.57   12.529%   $279.7133   $279.71   $279.70   $279.70  
  Purchase #2 $143.37   5.549%   $151.3256   $151.33   $151.35   $151.30  
  Purchase #3 $163.49   13.384%   $185.3715   $185.37   $185.35   $185.40  
  Purchase #4 $33.71   3.123%   $34.7628   $34.76   $34.75   $34.80  
  Purchase #5 $173.53   5.419%   $182.9336   $182.93   $182.95   $182.90  
  Purchase #6 $210.91   11.759%   $235.7109   $235.71   $235.70   $235.70  
  Purchase #7 $97.81   3.781%   $101.5082   $101.51   $101.50   $101.50  
  7 purchase totals = $1,071.39       $1,171.3259   $1,171.32   $1,171.30   $1,171.30