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 $244.75   12.252%   $274.7368   $274.74   $274.75   $274.70  
  Purchase #2 $97.65   4.493%   $102.0374   $102.04   $102.05   $102.00  
  Purchase #3 $12.50   13.491%   $14.1864   $14.19   $14.20   $14.20  
  Purchase #4 $230.90   8.386%   $250.2633   $250.26   $250.25   $250.30  
  Purchase #5 $107.93   6.691%   $115.1516   $115.15   $115.15   $115.20  
  Purchase #6 $187.99   6.566%   $200.3334   $200.33   $200.35   $200.30  
  Purchase #7 $161.38   11.777%   $180.3857   $180.39   $180.40   $180.40  
  7 purchase totals = $1,043.10       $1,137.0946   $1,137.10   $1,137.15   $1,137.10