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 $13.34   3.512%   $13.8085   $13.81   $13.80   $13.80  
  Purchase #2 $89.63   11.436%   $99.8801   $99.88   $99.90   $99.90  
  Purchase #3 $218.42   11.587%   $243.7283   $243.73   $243.75   $243.70  
  Purchase #4 $136.37   12.197%   $153.003   $153.00   $153.00   $153.00  
  Purchase #5 $231.25   10.734%   $256.0724   $256.07   $256.05   $256.10  
  Purchase #6 $103.25   13.673%   $117.3674   $117.37   $117.35   $117.40  
  Purchase #7 $7.69   6.812%   $8.2138   $8.21   $8.20   $8.20  
  7 purchase totals = $799.95       $892.0735   $892.07   $892.05   $892.10