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 $38.34   4.215%   $39.956   $39.96   $39.95   $40.00  
  Purchase #2 $5.98   7.405%   $6.4228   $6.42   $6.40   $6.40  
  Purchase #3 $100.55   7.397%   $107.9877   $107.99   $108.00   $108.00  
  Purchase #4 $234.87   6.387%   $249.8711   $249.87   $249.85   $249.90  
  Purchase #5 $49.61   3.506%   $51.3493   $51.35   $51.35   $51.30  
  Purchase #6 $86.67   12.843%   $97.801   $97.80   $97.80   $97.80  
  Purchase #7 $82.72   8.404%   $89.6718   $89.67   $89.65   $89.70  
  7 purchase totals = $598.74       $643.0597   $643.06   $643.00   $643.10