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 $5.90   7.838%   $6.3624   $6.36   $6.35   $6.40  
  Purchase #2 $130.51   5.529%   $137.7259   $137.73   $137.75   $137.70  
  Purchase #3 $191.97   4.202%   $200.0366   $200.04   $200.05   $200.00  
  Purchase #4 $46.90   12.390%   $52.7109   $52.71   $52.70   $52.70  
  Purchase #5 $153.99   13.124%   $174.1996   $174.20   $174.20   $174.20  
  Purchase #6 $79.40   7.419%   $85.2907   $85.29   $85.30   $85.30  
  Purchase #7 $168.68   8.534%   $183.0752   $183.08   $183.10   $183.10  
  7 purchase totals = $777.35       $839.4013   $839.41   $839.45   $839.40