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 $232.61   12.347%   $261.3304   $261.33   $261.35   $261.30  
  Purchase #2 $103.26   10.207%   $113.7997   $113.80   $113.80   $113.80  
  Purchase #3 $107.77   11.750%   $120.433   $120.43   $120.45   $120.40  
  Purchase #4 $60.62   13.781%   $68.974   $68.97   $68.95   $69.00  
  Purchase #5 $32.30   5.277%   $34.0045   $34.00   $34.00   $34.00  
  Purchase #6 $58.28   7.313%   $62.542   $62.54   $62.55   $62.50  
  Purchase #7 $131.61   12.380%   $147.9033   $147.90   $147.90   $147.90  
  7 purchase totals = $726.45       $808.9869   $808.97   $809.00   $808.90