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 $7.99   13.266%   $9.05   $9.05   $9.05   $9.10  
  Purchase #2 $230.81   12.511%   $259.6866   $259.69   $259.70   $259.70  
  Purchase #3 $170.32   4.850%   $178.5805   $178.58   $178.60   $178.60  
  Purchase #4 $158.60   3.536%   $164.2081   $164.21   $164.20   $164.20  
  Purchase #5 $20.27   6.737%   $21.6356   $21.64   $21.65   $21.60  
  Purchase #6 $152.32   8.847%   $165.7958   $165.80   $165.80   $165.80  
  Purchase #7 $169.47   3.194%   $174.8829   $174.88   $174.90   $174.90  
  7 purchase totals = $909.78       $973.8395   $973.85   $973.90   $973.90