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 $57.44   13.752%   $65.3391   $65.34   $65.35   $65.30  
  Purchase #2 $109.30   6.636%   $116.5531   $116.55   $116.55   $116.60  
  Purchase #3 $155.70   11.219%   $173.168   $173.17   $173.15   $173.20  
  Purchase #4 $84.44   12.262%   $94.794   $94.79   $94.80   $94.80  
  Purchase #5 $66.79   5.354%   $70.3659   $70.37   $70.35   $70.40  
  Purchase #6 $208.76   3.748%   $216.5843   $216.58   $216.60   $216.60  
  Purchase #7 $215.49   8.316%   $233.4101   $233.41   $233.40   $233.40  
  7 purchase totals = $897.92       $970.2145   $970.21   $970.20   $970.30