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 $12.25   7.394%   $13.1558   $13.16   $13.15   $13.20  
  Purchase #2 $150.93   13.668%   $171.5591   $171.56   $171.55   $171.60  
  Purchase #3 $224.29   8.491%   $243.3345   $243.33   $243.35   $243.30  
  Purchase #4 $219.41   11.130%   $243.8303   $243.83   $243.85   $243.80  
  Purchase #5 $16.58   3.531%   $17.1654   $17.17   $17.15   $17.20  
  Purchase #6 $77.96   6.190%   $82.7857   $82.79   $82.80   $82.80  
  Purchase #7 $140.99   11.334%   $156.9698   $156.97   $156.95   $157.00  
  7 purchase totals = $842.41       $928.8006   $928.81   $928.80   $928.90