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 $157.81   8.779%   $171.6641   $171.66   $171.65   $171.70  
  Purchase #2 $248.81   8.249%   $269.3343   $269.33   $269.35   $269.30  
  Purchase #3 $137.66   13.177%   $155.7995   $155.80   $155.80   $155.80  
  Purchase #4 $211.34   8.312%   $228.9066   $228.91   $228.90   $228.90  
  Purchase #5 $156.94   13.509%   $178.141   $178.14   $178.15   $178.10  
  Purchase #6 $57.27   12.147%   $64.2266   $64.23   $64.25   $64.20  
  Purchase #7 $222.39   5.226%   $234.0121   $234.01   $234.00   $234.00  
  7 purchase totals = $1,192.22       $1,302.0842   $1,302.08   $1,302.10   $1,302.00