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 $10.28   3.713%   $10.6617   $10.66   $10.65   $10.70  
  Purchase #2 $24.67   9.306%   $26.9658   $26.97   $26.95   $27.00  
  Purchase #3 $120.87   11.606%   $134.8982   $134.90   $134.90   $134.90  
  Purchase #4 $129.54   10.329%   $142.9202   $142.92   $142.90   $142.90  
  Purchase #5 $133.48   8.804%   $145.2316   $145.23   $145.25   $145.20  
  Purchase #6 $172.85   7.447%   $185.7221   $185.72   $185.70   $185.70  
  Purchase #7 $5.99   8.842%   $6.5196   $6.52   $6.50   $6.50  
  7 purchase totals = $597.68       $652.9192   $652.92   $652.85   $652.90