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 $99.32   9.736%   $108.9898   $108.99   $109.00   $109.00  
  Purchase #2 $128.26   8.450%   $139.098   $139.10   $139.10   $139.10  
  Purchase #3 $144.78   7.695%   $155.9208   $155.92   $155.90   $155.90  
  Purchase #4 $242.98   4.497%   $253.9068   $253.91   $253.90   $253.90  
  Purchase #5 $50.83   6.704%   $54.2376   $54.24   $54.25   $54.20  
  Purchase #6 $224.99   12.528%   $253.1767   $253.18   $253.20   $253.20  
  Purchase #7 $11.59   6.484%   $12.3415   $12.34   $12.35   $12.30  
  7 purchase totals = $902.75       $977.6712   $977.68   $977.70   $977.60