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 $227.87   6.296%   $242.2167   $242.22   $242.20   $242.20  
  Purchase #2 $183.33   4.158%   $190.9529   $190.95   $190.95   $191.00  
  Purchase #3 $170.42   4.563%   $178.1963   $178.20   $178.20   $178.20  
  Purchase #4 $209.28   5.253%   $220.2735   $220.27   $220.25   $220.30  
  Purchase #5 $178.88   8.200%   $193.5482   $193.55   $193.55   $193.50  
  Purchase #6 $189.42   9.734%   $207.8581   $207.86   $207.85   $207.90  
  Purchase #7 $20.97   9.368%   $22.9345   $22.93   $22.95   $22.90  
  7 purchase totals = $1,180.17       $1,255.9802   $1,255.98   $1,255.95   $1,256.00