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 $126.57   6.426%   $134.7034   $134.70   $134.70   $134.70  
  Purchase #2 $23.29   10.818%   $25.8095   $25.81   $25.80   $25.80  
  Purchase #3 $54.92   4.309%   $57.2865   $57.29   $57.30   $57.30  
  Purchase #4 $174.35   6.188%   $185.1388   $185.14   $185.15   $185.10  
  Purchase #5 $92.34   3.792%   $95.8415   $95.84   $95.85   $95.80  
  Purchase #6 $73.71   3.296%   $76.1395   $76.14   $76.15   $76.10  
  Purchase #7 $12.68   13.619%   $14.4069   $14.41   $14.40   $14.40  
  7 purchase totals = $557.86       $589.3261   $589.33   $589.35   $589.20