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 $17.59   3.846%   $18.2665   $18.27   $18.25   $18.30  
  Purchase #2 $69.90   9.125%   $76.2784   $76.28   $76.30   $76.30  
  Purchase #3 $11.96   9.122%   $13.051   $13.05   $13.05   $13.10  
  Purchase #4 $193.73   13.204%   $219.3101   $219.31   $219.30   $219.30  
  Purchase #5 $180.31   10.297%   $198.8765   $198.88   $198.90   $198.90  
  Purchase #6 $203.25   4.224%   $211.8353   $211.84   $211.85   $211.80  
  Purchase #7 $96.91   7.143%   $103.8323   $103.83   $103.85   $103.80  
  7 purchase totals = $773.65       $841.4501   $841.46   $841.50   $841.50