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 $21.64   10.481%   $23.9081   $23.91   $23.90   $23.90  
  Purchase #2 $80.27   13.242%   $90.8994   $90.90   $90.90   $90.90  
  Purchase #3 $89.35   6.746%   $95.3776   $95.38   $95.40   $95.40  
  Purchase #4 $57.90   3.596%   $59.9821   $59.98   $60.00   $60.00  
  Purchase #5 $70.25   8.823%   $76.4482   $76.45   $76.45   $76.40  
  Purchase #6 $24.53   7.233%   $26.3043   $26.30   $26.30   $26.30  
  Purchase #7 $32.80   9.103%   $35.7858   $35.79   $35.80   $35.80  
  7 purchase totals = $376.74       $408.7055   $408.71   $408.75   $408.70