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 $18.30   12.416%   $20.5721   $20.57   $20.55   $20.60  
  Purchase #2 $142.50   3.589%   $147.6143   $147.61   $147.60   $147.60  
  Purchase #3 $100.55   9.421%   $110.0228   $110.02   $110.00   $110.00  
  Purchase #4 $45.95   11.400%   $51.1883   $51.19   $51.20   $51.20  
  Purchase #5 $21.49   12.563%   $24.1898   $24.19   $24.20   $24.20  
  Purchase #6 $45.89   9.136%   $50.0825   $50.08   $50.10   $50.10  
  Purchase #7 $78.65   3.733%   $81.586   $81.59   $81.60   $81.60  
  7 purchase totals = $453.33       $485.2558   $485.25   $485.25   $485.30