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 $93.65   6.168%   $99.4263   $99.43   $99.45   $99.40  
  Purchase #2 $114.51   4.498%   $119.6607   $119.66   $119.65   $119.70  
  Purchase #3 $95.83   6.613%   $102.1672   $102.17   $102.15   $102.20  
  Purchase #4 $236.35   6.730%   $252.2564   $252.26   $252.25   $252.30  
  Purchase #5 $152.41   8.326%   $165.0997   $165.10   $165.10   $165.10  
  Purchase #6 $77.84   6.312%   $82.7533   $82.75   $82.75   $82.80  
  Purchase #7 $153.35   6.503%   $163.3224   $163.32   $163.30   $163.30  
  7 purchase totals = $923.94       $984.6860   $984.69   $984.65   $984.80