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 $136.75   13.560%   $155.2933   $155.29   $155.30   $155.30  
  Purchase #2 $40.49   8.827%   $44.0641   $44.06   $44.05   $44.10  
  Purchase #3 $76.96   3.362%   $79.5474   $79.55   $79.55   $79.50  
  Purchase #4 $47.85   6.387%   $50.9062   $50.91   $50.90   $50.90  
  Purchase #5 $247.87   6.482%   $263.9369   $263.94   $263.95   $263.90  
  Purchase #6 $123.57   5.745%   $130.6691   $130.67   $130.65   $130.70  
  Purchase #7 $210.47   10.197%   $231.9316   $231.93   $231.95   $231.90  
  7 purchase totals = $883.96       $956.3486   $956.35   $956.35   $956.30