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 $233.51   12.146%   $261.8721   $261.87   $261.85   $261.90  
  Purchase #2 $16.51   13.205%   $18.6901   $18.69   $18.70   $18.70  
  Purchase #3 $58.30   11.342%   $64.9124   $64.91   $64.90   $64.90  
  Purchase #4 $113.35   5.294%   $119.3507   $119.35   $119.35   $119.40  
  Purchase #5 $146.78   8.339%   $159.02   $159.02   $159.00   $159.00  
  Purchase #6 $33.62   3.534%   $34.8081   $34.81   $34.80   $34.80  
  Purchase #7 $117.57   4.826%   $123.2439   $123.24   $123.25   $123.20  
  7 purchase totals = $719.64       $781.8973   $781.89   $781.85   $781.90