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 $54.49   10.731%   $60.3373   $60.34   $60.35   $60.30  
  Purchase #2 $101.93   4.374%   $106.3884   $106.39   $106.40   $106.40  
  Purchase #3 $84.70   7.787%   $91.2956   $91.30   $91.30   $91.30  
  Purchase #4 $177.36   4.458%   $185.2667   $185.27   $185.25   $185.30  
  Purchase #5 $37.68   13.690%   $42.8384   $42.84   $42.85   $42.80  
  Purchase #6 $117.33   4.721%   $122.8691   $122.87   $122.85   $122.90  
  Purchase #7 $8.75   5.197%   $9.2047   $9.20   $9.20   $9.20  
  7 purchase totals = $582.24       $618.2002   $618.21   $618.20   $618.20