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 $20.51   4.686%   $21.4711   $21.47   $21.45   $21.50  
  Purchase #2 $64.55   7.416%   $69.337   $69.34   $69.35   $69.30  
  Purchase #3 $102.44   6.789%   $109.3947   $109.39   $109.40   $109.40  
  Purchase #4 $222.97   10.332%   $246.0073   $246.01   $246.00   $246.00  
  Purchase #5 $31.33   8.325%   $33.9382   $33.94   $33.95   $33.90  
  Purchase #6 $64.89   8.533%   $70.4271   $70.43   $70.45   $70.40  
  Purchase #7 $116.53   3.126%   $120.1727   $120.17   $120.15   $120.20  
  7 purchase totals = $623.22       $670.7481   $670.75   $670.75   $670.70