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 $122.81   10.871%   $136.1607   $136.16   $136.15   $136.20  
  Purchase #2 $233.31   6.572%   $248.6431   $248.64   $248.65   $248.60  
  Purchase #3 $57.47   7.365%   $61.7027   $61.70   $61.70   $61.70  
  Purchase #4 $182.78   12.233%   $205.1395   $205.14   $205.15   $205.10  
  Purchase #5 $86.59   8.754%   $94.1701   $94.17   $94.15   $94.20  
  Purchase #6 $138.77   6.571%   $147.8886   $147.89   $147.90   $147.90  
  Purchase #7 $103.93   11.779%   $116.1719   $116.17   $116.15   $116.20  
  7 purchase totals = $925.66       $1,009.8766   $1,009.87   $1,009.85   $1,009.90