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 $9.55   3.764%   $9.9095   $9.91   $9.90   $9.90  
  Purchase #2 $153.32   3.144%   $158.1404   $158.14   $158.15   $158.10  
  Purchase #3 $53.31   12.175%   $59.8005   $59.80   $59.80   $59.80  
  Purchase #4 $174.94   5.403%   $184.392   $184.39   $184.40   $184.40  
  Purchase #5 $20.80   3.528%   $21.5338   $21.53   $21.55   $21.50  
  Purchase #6 $28.55   9.839%   $31.359   $31.36   $31.35   $31.40  
  Purchase #7 $8.97   3.156%   $9.2531   $9.25   $9.25   $9.30  
  7 purchase totals = $449.44       $474.3883   $474.38   $474.40   $474.40