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 $80.81   9.586%   $88.5564   $88.56   $88.55   $88.60  
  Purchase #2 $126.58   6.611%   $134.9482   $134.95   $134.95   $134.90  
  Purchase #3 $239.36   13.870%   $272.5592   $272.56   $272.55   $272.60  
  Purchase #4 $20.73   11.356%   $23.0841   $23.08   $23.10   $23.10  
  Purchase #5 $88.29   10.521%   $97.579   $97.58   $97.60   $97.60  
  Purchase #6 $98.25   13.116%   $111.1365   $111.14   $111.15   $111.10  
  Purchase #7 $125.93   3.336%   $130.131   $130.13   $130.15   $130.10  
  7 purchase totals = $779.95       $857.9944   $858.00   $858.05   $858.00