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 $79.63   12.846%   $89.8593   $89.86   $89.85   $89.90  
  Purchase #2 $130.90   12.769%   $147.6146   $147.61   $147.60   $147.60  
  Purchase #3 $97.52   13.451%   $110.6374   $110.64   $110.65   $110.60  
  Purchase #4 $14.80   10.491%   $16.3527   $16.35   $16.35   $16.40  
  Purchase #5 $103.58   4.400%   $108.1375   $108.14   $108.15   $108.10  
  Purchase #6 $66.60   8.604%   $72.3303   $72.33   $72.35   $72.30  
  Purchase #7 $27.79   10.700%   $30.7635   $30.76   $30.75   $30.80  
  7 purchase totals = $520.82       $575.6953   $575.69   $575.70   $575.70