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 $241.62   10.687%   $267.4419   $267.44   $267.45   $267.40  
  Purchase #2 $70.87   6.636%   $75.5729   $75.57   $75.55   $75.60  
  Purchase #3 $80.58   3.481%   $83.385   $83.39   $83.40   $83.40  
  Purchase #4 $69.76   6.211%   $74.0928   $74.09   $74.10   $74.10  
  Purchase #5 $106.35   13.651%   $120.8678   $120.87   $120.85   $120.90  
  Purchase #6 $12.35   12.352%   $13.8755   $13.88   $13.90   $13.90  
  Purchase #7 $168.47   9.422%   $184.3432   $184.34   $184.35   $184.30  
  7 purchase totals = $750.00       $819.5791   $819.58   $819.60   $819.60