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 $99.69   10.401%   $110.0588   $110.06   $110.05   $110.10  
  Purchase #2 $110.34   12.780%   $124.4415   $124.44   $124.45   $124.40  
  Purchase #3 $210.34   4.657%   $220.1355   $220.14   $220.15   $220.10  
  Purchase #4 $12.48   8.285%   $13.514   $13.51   $13.50   $13.50  
  Purchase #5 $184.34   8.246%   $199.5407   $199.54   $199.55   $199.50  
  Purchase #6 $138.94   13.514%   $157.7164   $157.72   $157.70   $157.70  
  Purchase #7 $12.77   9.360%   $13.9653   $13.97   $13.95   $14.00  
  7 purchase totals = $768.90       $839.3722   $839.38   $839.35   $839.30