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 $30.69   12.505%   $34.5278   $34.53   $34.55   $34.50  
  Purchase #2 $124.49   9.440%   $136.2419   $136.24   $136.25   $136.20  
  Purchase #3 $135.31   9.171%   $147.7193   $147.72   $147.70   $147.70  
  Purchase #4 $162.44   11.659%   $181.3789   $181.38   $181.40   $181.40  
  Purchase #5 $235.41   6.495%   $250.6999   $250.70   $250.70   $250.70  
  Purchase #6 $78.70   9.508%   $86.1828   $86.18   $86.20   $86.20  
  Purchase #7 $62.92   5.690%   $66.5001   $66.50   $66.50   $66.50  
  7 purchase totals = $829.96       $903.2507   $903.25   $903.30   $903.20