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 $84.61   5.584%   $89.3346   $89.33   $89.35   $89.30  
  Purchase #2 $118.81   10.182%   $130.9072   $130.91   $130.90   $130.90  
  Purchase #3 $20.52   7.184%   $21.9942   $21.99   $22.00   $22.00  
  Purchase #4 $168.49   3.323%   $174.0889   $174.09   $174.10   $174.10  
  Purchase #5 $158.35   4.192%   $164.988   $164.99   $165.00   $165.00  
  Purchase #6 $38.79   11.504%   $43.2524   $43.25   $43.25   $43.30  
  Purchase #7 $245.44   11.624%   $273.9699   $273.97   $273.95   $274.00  
  7 purchase totals = $835.01       $898.5352   $898.53   $898.55   $898.60