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 $248.46   12.591%   $279.7436   $279.74   $279.75   $279.70  
  Purchase #2 $42.44   9.312%   $46.392   $46.39   $46.40   $46.40  
  Purchase #3 $176.95   13.845%   $201.4487   $201.45   $201.45   $201.40  
  Purchase #4 $221.66   12.548%   $249.4739   $249.47   $249.45   $249.50  
  Purchase #5 $244.45   9.447%   $267.5432   $267.54   $267.55   $267.50  
  Purchase #6 $44.78   13.805%   $50.9619   $50.96   $50.95   $51.00  
  Purchase #7 $51.85   9.387%   $56.7172   $56.72   $56.70   $56.70  
  7 purchase totals = $1,030.59       $1,152.2805   $1,152.27   $1,152.25   $1,152.20