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 $7.31   6.845%   $7.8104   $7.81   $7.80   $7.80  
  Purchase #2 $27.68   6.144%   $29.3807   $29.38   $29.40   $29.40  
  Purchase #3 $171.66   3.227%   $177.1995   $177.20   $177.20   $177.20  
  Purchase #4 $90.27   5.475%   $95.2123   $95.21   $95.20   $95.20  
  Purchase #5 $198.84   11.313%   $221.3348   $221.33   $221.35   $221.30  
  Purchase #6 $92.93   7.569%   $99.9639   $99.96   $99.95   $100.00  
  Purchase #7 $21.26   4.680%   $22.255   $22.26   $22.25   $22.30  
  7 purchase totals = $609.95       $653.1566   $653.15   $653.15   $653.20