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 $12.72   3.129%   $13.118   $13.12   $13.10   $13.10  
  Purchase #2 $122.71   6.375%   $130.5328   $130.53   $130.55   $130.50  
  Purchase #3 $113.77   4.670%   $119.0831   $119.08   $119.10   $119.10  
  Purchase #4 $44.87   12.636%   $50.5398   $50.54   $50.55   $50.50  
  Purchase #5 $227.84   7.244%   $244.3447   $244.34   $244.35   $244.30  
  Purchase #6 $21.69   12.729%   $24.4509   $24.45   $24.45   $24.50  
  Purchase #7 $6.29   4.676%   $6.5841   $6.58   $6.60   $6.60  
  7 purchase totals = $549.89       $588.6534   $588.64   $588.70   $588.60