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 $128.35   7.716%   $138.2535   $138.25   $138.25   $138.30  
  Purchase #2 $5.44   11.162%   $6.0472   $6.05   $6.05   $6.00  
  Purchase #3 $140.70   10.441%   $155.3905   $155.39   $155.40   $155.40  
  Purchase #4 $9.72   3.740%   $10.0835   $10.08   $10.10   $10.10  
  Purchase #5 $27.79   9.780%   $30.5079   $30.51   $30.50   $30.50  
  Purchase #6 $115.35   11.121%   $128.1781   $128.18   $128.20   $128.20  
  Purchase #7 $42.62   4.137%   $44.3832   $44.38   $44.40   $44.40  
  7 purchase totals = $469.97       $512.8439   $512.84   $512.90   $512.90