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 $249.91   7.802%   $269.408   $269.41   $269.40   $269.40  
  Purchase #2 $14.79   3.642%   $15.3287   $15.33   $15.35   $15.30  
  Purchase #3 $229.83   13.123%   $259.9906   $259.99   $260.00   $260.00  
  Purchase #4 $220.34   12.365%   $247.585   $247.59   $247.60   $247.60  
  Purchase #5 $245.86   13.688%   $279.5133   $279.51   $279.50   $279.50  
  Purchase #6 $106.95   12.592%   $120.4171   $120.42   $120.40   $120.40  
  Purchase #7 $66.90   7.560%   $71.9576   $71.96   $71.95   $72.00  
  7 purchase totals = $1,134.58       $1,264.2003   $1,264.21   $1,264.20   $1,264.20