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 $85.52   5.440%   $90.1723   $90.17   $90.15   $90.20  
  Purchase #2 $103.76   5.333%   $109.2935   $109.29   $109.30   $109.30  
  Purchase #3 $172.70   9.871%   $189.7472   $189.75   $189.75   $189.70  
  Purchase #4 $130.66   3.609%   $135.3755   $135.38   $135.40   $135.40  
  Purchase #5 $13.92   6.381%   $14.8082   $14.81   $14.80   $14.80  
  Purchase #6 $106.35   3.470%   $110.0403   $110.04   $110.05   $110.00  
  Purchase #7 $27.77   9.225%   $30.3318   $30.33   $30.35   $30.30  
  7 purchase totals = $640.68       $679.7688   $679.77   $679.80   $679.70