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 $35.79   11.737%   $39.9907   $39.99   $40.00   $40.00  
  Purchase #2 $103.26   9.815%   $113.395   $113.40   $113.40   $113.40  
  Purchase #3 $97.99   12.235%   $109.9791   $109.98   $110.00   $110.00  
  Purchase #4 $66.61   5.714%   $70.4161   $70.42   $70.40   $70.40  
  Purchase #5 $38.27   9.754%   $42.0029   $42.00   $42.00   $42.00  
  Purchase #6 $142.67   13.850%   $162.4298   $162.43   $162.45   $162.40  
  Purchase #7 $67.48   9.426%   $73.8407   $73.84   $73.85   $73.80  
  7 purchase totals = $552.07       $612.0543   $612.06   $612.10   $612.00