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 $7.72   8.585%   $8.3828   $8.38   $8.40   $8.40  
  Purchase #2 $179.70   5.496%   $189.5763   $189.58   $189.60   $189.60  
  Purchase #3 $68.74   4.195%   $71.6236   $71.62   $71.60   $71.60  
  Purchase #4 $13.28   12.395%   $14.9261   $14.93   $14.95   $14.90  
  Purchase #5 $223.41   7.338%   $239.8038   $239.80   $239.80   $239.80  
  Purchase #6 $188.35   13.782%   $214.3084   $214.31   $214.30   $214.30  
  Purchase #7 $238.73   10.632%   $264.1118   $264.11   $264.10   $264.10  
  7 purchase totals = $919.93       $1,002.7328   $1,002.73   $1,002.75   $1,002.70