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 $28.34   8.636%   $30.7874   $30.79   $30.80   $30.80  
  Purchase #2 $157.47   10.557%   $174.0941   $174.09   $174.10   $174.10  
  Purchase #3 $157.86   12.704%   $177.9145   $177.91   $177.90   $177.90  
  Purchase #4 $92.80   11.320%   $103.305   $103.31   $103.30   $103.30  
  Purchase #5 $111.68   13.811%   $127.1041   $127.10   $127.10   $127.10  
  Purchase #6 $176.64   9.649%   $193.684   $193.68   $193.70   $193.70  
  Purchase #7 $147.94   11.420%   $164.8347   $164.83   $164.85   $164.80  
  7 purchase totals = $872.73       $971.7238   $971.71   $971.75   $971.70