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 $250.47   13.401%   $284.0355   $284.04   $284.05   $284.00  
  Purchase #2 $247.31   11.775%   $276.4308   $276.43   $276.45   $276.40  
  Purchase #3 $117.66   6.195%   $124.949   $124.95   $124.95   $124.90  
  Purchase #4 $70.48   6.708%   $75.2078   $75.21   $75.20   $75.20  
  Purchase #5 $89.97   6.592%   $95.9008   $95.90   $95.90   $95.90  
  Purchase #6 $87.90   12.594%   $98.9701   $98.97   $98.95   $99.00  
  Purchase #7 $122.37   6.544%   $130.3779   $130.38   $130.40   $130.40  
  7 purchase totals = $986.16       $1,085.8719   $1,085.88   $1,085.90   $1,085.80