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 $19.39   7.791%   $20.9007   $20.90   $20.90   $20.90  
  Purchase #2 $11.59   10.819%   $12.8439   $12.84   $12.85   $12.80  
  Purchase #3 $192.81   12.377%   $216.6741   $216.67   $216.65   $216.70  
  Purchase #4 $101.47   12.640%   $114.2958   $114.30   $114.30   $114.30  
  Purchase #5 $11.99   12.581%   $13.4985   $13.50   $13.50   $13.50  
  Purchase #6 $45.45   9.649%   $49.8355   $49.84   $49.85   $49.80  
  Purchase #7 $193.40   10.849%   $214.382   $214.38   $214.40   $214.40  
  7 purchase totals = $576.10       $642.4305   $642.43   $642.45   $642.40