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 $20.40   5.327%   $21.4867   $21.49   $21.50   $21.50  
  Purchase #2 $183.35   3.237%   $189.285   $189.29   $189.30   $189.30  
  Purchase #3 $102.88   12.386%   $115.6227   $115.62   $115.60   $115.60  
  Purchase #4 $181.78   10.582%   $201.016   $201.02   $201.00   $201.00  
  Purchase #5 $146.99   5.777%   $155.4816   $155.48   $155.50   $155.50  
  Purchase #6 $105.34   4.333%   $109.9044   $109.90   $109.90   $109.90  
  Purchase #7 $23.74   11.270%   $26.4155   $26.42   $26.40   $26.40  
  7 purchase totals = $764.48       $819.2119   $819.22   $819.20   $819.20