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 $238.59   13.149%   $269.9622   $269.96   $269.95   $270.00  
  Purchase #2 $155.79   13.208%   $176.3667   $176.37   $176.35   $176.40  
  Purchase #3 $36.29   9.477%   $39.7292   $39.73   $39.75   $39.70  
  Purchase #4 $206.92   6.608%   $220.5933   $220.59   $220.60   $220.60  
  Purchase #5 $99.33   13.379%   $112.6194   $112.62   $112.60   $112.60  
  Purchase #6 $224.82   8.357%   $243.6082   $243.61   $243.60   $243.60  
  Purchase #7 $246.75   10.106%   $271.6866   $271.69   $271.70   $271.70  
  7 purchase totals = $1,208.49       $1,334.5656   $1,334.57   $1,334.55   $1,334.60