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 $130.41   6.422%   $138.7849   $138.78   $138.80   $138.80  
  Purchase #2 $19.40   12.554%   $21.8355   $21.84   $21.85   $21.80  
  Purchase #3 $195.55   9.463%   $214.0549   $214.05   $214.05   $214.10  
  Purchase #4 $87.49   4.583%   $91.4997   $91.50   $91.50   $91.50  
  Purchase #5 $151.75   5.772%   $160.509   $160.51   $160.50   $160.50  
  Purchase #6 $77.58   13.191%   $87.8136   $87.81   $87.80   $87.80  
  Purchase #7 $129.71   4.277%   $135.2577   $135.26   $135.25   $135.30  
  7 purchase totals = $791.89       $849.7553   $849.75   $849.75   $849.80