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 $9.85   12.151%   $11.0469   $11.05   $11.05   $11.00  
  Purchase #2 $31.85   7.864%   $34.3547   $34.35   $34.35   $34.40  
  Purchase #3 $90.77   6.289%   $96.4785   $96.48   $96.50   $96.50  
  Purchase #4 $75.62   11.667%   $84.4426   $84.44   $84.45   $84.40  
  Purchase #5 $77.89   3.472%   $80.5943   $80.59   $80.60   $80.60  
  Purchase #6 $188.45   3.861%   $195.7261   $195.73   $195.75   $195.70  
  Purchase #7 $56.58   4.308%   $59.0175   $59.02   $59.00   $59.00  
  7 purchase totals = $531.01       $561.6606   $561.66   $561.70   $561.60