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 $136.35   13.420%   $154.6482   $154.65   $154.65   $154.60  
  Purchase #2 $181.95   3.448%   $188.2236   $188.22   $188.20   $188.20  
  Purchase #3 $214.40   3.634%   $222.1913   $222.19   $222.20   $222.20  
  Purchase #4 $38.74   9.251%   $42.3238   $42.32   $42.30   $42.30  
  Purchase #5 $35.63   5.472%   $37.5797   $37.58   $37.60   $37.60  
  Purchase #6 $37.79   11.508%   $42.1389   $42.14   $42.15   $42.10  
  Purchase #7 $120.84   7.514%   $129.9199   $129.92   $129.90   $129.90  
  7 purchase totals = $765.70       $817.0254   $817.02   $817.00   $816.90