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 $17.70   13.362%   $20.0651   $20.07   $20.05   $20.10  
  Purchase #2 $6.93   8.788%   $7.539   $7.54   $7.55   $7.50  
  Purchase #3 $221.75   12.515%   $249.502   $249.50   $249.50   $249.50  
  Purchase #4 $74.56   7.833%   $80.4003   $80.40   $80.40   $80.40  
  Purchase #5 $163.48   6.191%   $173.601   $173.60   $173.60   $173.60  
  Purchase #6 $21.59   12.817%   $24.3572   $24.36   $24.35   $24.40  
  Purchase #7 $155.50   3.705%   $161.2613   $161.26   $161.25   $161.30  
  7 purchase totals = $661.51       $716.7259   $716.73   $716.70   $716.80