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 $221.92   11.713%   $247.9135   $247.91   $247.90   $247.90  
  Purchase #2 $19.76   12.224%   $22.1755   $22.18   $22.20   $22.20  
  Purchase #3 $150.26   5.382%   $158.347   $158.35   $158.35   $158.30  
  Purchase #4 $248.56   5.152%   $261.3658   $261.37   $261.35   $261.40  
  Purchase #5 $250.82   6.711%   $267.6525   $267.65   $267.65   $267.70  
  Purchase #6 $202.72   5.515%   $213.9   $213.90   $213.90   $213.90  
  Purchase #7 $148.66   6.553%   $158.4017   $158.40   $158.40   $158.40  
  7 purchase totals = $1,242.70       $1,329.7560   $1,329.76   $1,329.75   $1,329.80