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 $245.47   13.671%   $279.0282   $279.03   $279.05   $279.00  
  Purchase #2 $119.35   5.652%   $126.0957   $126.10   $126.10   $126.10  
  Purchase #3 $73.35   11.161%   $81.5366   $81.54   $81.55   $81.50  
  Purchase #4 $143.70   12.663%   $161.8967   $161.90   $161.90   $161.90  
  Purchase #5 $217.34   10.661%   $240.5106   $240.51   $240.50   $240.50  
  Purchase #6 $44.46   12.785%   $50.1442   $50.14   $50.15   $50.10  
  Purchase #7 $60.82   6.814%   $64.9643   $64.96   $64.95   $65.00  
  7 purchase totals = $904.49       $1,004.1763   $1,004.18   $1,004.20   $1,004.10