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 $100.71   8.607%   $109.3781   $109.38   $109.40   $109.40  
  Purchase #2 $113.65   9.627%   $124.5911   $124.59   $124.60   $124.60  
  Purchase #3 $8.36   12.867%   $9.4357   $9.44   $9.45   $9.40  
  Purchase #4 $79.46   6.457%   $84.5907   $84.59   $84.60   $84.60  
  Purchase #5 $224.56   7.791%   $242.0555   $242.06   $242.05   $242.10  
  Purchase #6 $242.27   11.776%   $270.7997   $270.80   $270.80   $270.80  
  Purchase #7 $10.67   9.452%   $11.6785   $11.68   $11.70   $11.70  
  7 purchase totals = $779.68       $852.5293   $852.54   $852.60   $852.60