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 $12.36   6.589%   $13.1744   $13.17   $13.15   $13.20  
  Purchase #2 $122.55   6.124%   $130.055   $130.06   $130.05   $130.10  
  Purchase #3 $219.90   11.462%   $245.1049   $245.10   $245.10   $245.10  
  Purchase #4 $186.84   12.548%   $210.2847   $210.28   $210.30   $210.30  
  Purchase #5 $46.48   8.275%   $50.3262   $50.33   $50.35   $50.30  
  Purchase #6 $70.26   3.694%   $72.8554   $72.86   $72.85   $72.90  
  Purchase #7 $20.74   4.256%   $21.6227   $21.62   $21.60   $21.60  
  7 purchase totals = $679.13       $743.4233   $743.42   $743.40   $743.50