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 $18.70   7.507%   $20.1038   $20.10   $20.10   $20.10  
  Purchase #2 $66.26   5.643%   $69.9991   $70.00   $70.00   $70.00  
  Purchase #3 $118.48   13.316%   $134.2568   $134.26   $134.25   $134.30  
  Purchase #4 $220.42   8.531%   $239.224   $239.22   $239.20   $239.20  
  Purchase #5 $152.62   12.445%   $171.6136   $171.61   $171.60   $171.60  
  Purchase #6 $16.69   11.241%   $18.5661   $18.57   $18.55   $18.60  
  Purchase #7 $138.50   11.176%   $153.9788   $153.98   $154.00   $154.00  
  7 purchase totals = $731.67       $807.7422   $807.74   $807.70   $807.80