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 $90.52   9.718%   $99.3167   $99.32   $99.30   $99.30  
  Purchase #2 $66.30   7.812%   $71.4794   $71.48   $71.50   $71.50  
  Purchase #3 $102.54   4.498%   $107.1522   $107.15   $107.15   $107.20  
  Purchase #4 $164.99   10.564%   $182.4195   $182.42   $182.40   $182.40  
  Purchase #5 $15.65   4.410%   $16.3402   $16.34   $16.35   $16.30  
  Purchase #6 $127.99   13.574%   $145.3634   $145.36   $145.35   $145.40  
  Purchase #7 $21.66   3.631%   $22.4465   $22.45   $22.45   $22.40  
  7 purchase totals = $589.65       $644.5179   $644.52   $644.50   $644.50