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 $185.37   12.710%   $208.9305   $208.93   $208.95   $208.90  
  Purchase #2 $149.97   11.148%   $166.6887   $166.69   $166.70   $166.70  
  Purchase #3 $74.50   7.639%   $80.1911   $80.19   $80.20   $80.20  
  Purchase #4 $83.29   9.872%   $91.5124   $91.51   $91.50   $91.50  
  Purchase #5 $168.35   13.233%   $190.6278   $190.63   $190.65   $190.60  
  Purchase #6 $246.29   9.247%   $269.0644   $269.06   $269.05   $269.10  
  Purchase #7 $82.54   10.456%   $91.1704   $91.17   $91.15   $91.20  
  7 purchase totals = $990.31       $1,098.1853   $1,098.18   $1,098.20   $1,098.20