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 $6.92   12.768%   $7.8035   $7.80   $7.80   $7.80  
  Purchase #2 $5.37   7.298%   $5.7619   $5.76   $5.75   $5.80  
  Purchase #3 $103.69   5.412%   $109.3017   $109.30   $109.30   $109.30  
  Purchase #4 $42.63   3.313%   $44.0423   $44.04   $44.05   $44.00  
  Purchase #5 $55.57   3.110%   $57.2982   $57.30   $57.30   $57.30  
  Purchase #6 $158.90   11.535%   $177.2291   $177.23   $177.25   $177.20  
  Purchase #7 $110.76   11.855%   $123.8906   $123.89   $123.90   $123.90  
  7 purchase totals = $483.84       $525.3273   $525.32   $525.35   $525.30