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 $77.73   4.370%   $81.1268   $81.13   $81.15   $81.10  
  Purchase #2 $212.50   10.456%   $234.719   $234.72   $234.70   $234.70  
  Purchase #3 $91.57   9.868%   $100.6061   $100.61   $100.60   $100.60  
  Purchase #4 $13.77   11.383%   $15.3374   $15.34   $15.35   $15.30  
  Purchase #5 $33.91   12.331%   $38.0914   $38.09   $38.10   $38.10  
  Purchase #6 $118.67   8.176%   $128.3725   $128.37   $128.35   $128.40  
  Purchase #7 $45.93   7.713%   $49.4726   $49.47   $49.45   $49.50  
  7 purchase totals = $594.08       $647.7258   $647.73   $647.70   $647.70