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 $36.58   9.299%   $39.9816   $39.98   $40.00   $40.00  
  Purchase #2 $129.61   12.141%   $145.346   $145.35   $145.35   $145.30  
  Purchase #3 $134.31   13.519%   $152.4674   $152.47   $152.45   $152.50  
  Purchase #4 $203.91   5.556%   $215.2392   $215.24   $215.25   $215.20  
  Purchase #5 $211.26   4.363%   $220.4773   $220.48   $220.50   $220.50  
  Purchase #6 $35.41   8.192%   $38.3108   $38.31   $38.30   $38.30  
  Purchase #7 $38.55   13.638%   $43.8074   $43.81   $43.80   $43.80  
  7 purchase totals = $789.63       $855.6297   $855.64   $855.65   $855.60