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 $249.84   10.467%   $275.9908   $275.99   $276.00   $276.00  
  Purchase #2 $172.32   12.788%   $194.3563   $194.36   $194.35   $194.40  
  Purchase #3 $15.42   10.187%   $16.9908   $16.99   $17.00   $17.00  
  Purchase #4 $192.25   6.820%   $205.3615   $205.36   $205.35   $205.40  
  Purchase #5 $20.35   3.416%   $21.0452   $21.05   $21.05   $21.00  
  Purchase #6 $36.38   10.674%   $40.2632   $40.26   $40.25   $40.30  
  Purchase #7 $228.67   10.195%   $251.9829   $251.98   $252.00   $252.00  
  7 purchase totals = $915.23       $1,005.9907   $1,005.99   $1,006.00   $1,006.10