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 $169.40   10.179%   $186.6432   $186.64   $186.65   $186.60  
  Purchase #2 $120.63   7.113%   $129.2104   $129.21   $129.20   $129.20  
  Purchase #3 $112.58   9.532%   $123.3111   $123.31   $123.30   $123.30  
  Purchase #4 $8.41   12.315%   $9.4457   $9.45   $9.45   $9.40  
  Purchase #5 $80.67   10.608%   $89.2275   $89.23   $89.25   $89.20  
  Purchase #6 $74.51   3.203%   $76.8966   $76.90   $76.90   $76.90  
  Purchase #7 $23.50   7.710%   $25.3119   $25.31   $25.30   $25.30  
  7 purchase totals = $589.70       $640.0464   $640.05   $640.05   $639.90