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 $70.75   13.358%   $80.2008   $80.20   $80.20   $80.20  
  Purchase #2 $119.54   12.365%   $134.3211   $134.32   $134.30   $134.30  
  Purchase #3 $69.44   4.733%   $72.7266   $72.73   $72.75   $72.70  
  Purchase #4 $26.94   9.354%   $29.46   $29.46   $29.45   $29.50  
  Purchase #5 $199.34   9.191%   $217.6613   $217.66   $217.65   $217.70  
  Purchase #6 $130.95   4.478%   $136.8139   $136.81   $136.80   $136.80  
  Purchase #7 $191.45   3.464%   $198.0818   $198.08   $198.10   $198.10  
  7 purchase totals = $808.41       $869.2655   $869.26   $869.25   $869.30