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 $8.69   3.426%   $8.9877   $8.99   $9.00   $9.00  
  Purchase #2 $102.59   12.228%   $115.1347   $115.13   $115.15   $115.10  
  Purchase #3 $130.67   6.177%   $138.7415   $138.74   $138.75   $138.70  
  Purchase #4 $95.32   11.107%   $105.9072   $105.91   $105.90   $105.90  
  Purchase #5 $218.82   8.561%   $237.5532   $237.55   $237.55   $237.60  
  Purchase #6 $241.82   13.504%   $274.4754   $274.48   $274.50   $274.50  
  Purchase #7 $182.30   5.855%   $192.9737   $192.97   $192.95   $193.00  
  7 purchase totals = $980.21       $1,073.7734   $1,073.77   $1,073.80   $1,073.80