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 $238.82   12.655%   $269.0427   $269.04   $269.05   $269.00  
  Purchase #2 $183.83   10.622%   $203.3564   $203.36   $203.35   $203.40  
  Purchase #3 $245.32   5.793%   $259.5314   $259.53   $259.55   $259.50  
  Purchase #4 $215.99   8.291%   $233.8977   $233.90   $233.90   $233.90  
  Purchase #5 $169.54   6.738%   $180.9636   $180.96   $180.95   $181.00  
  Purchase #6 $223.97   12.860%   $252.7725   $252.77   $252.75   $252.80  
  Purchase #7 $218.53   11.104%   $242.7956   $242.80   $242.80   $242.80  
  7 purchase totals = $1,496.00       $1,642.3599   $1,642.36   $1,642.35   $1,642.40