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 $238.99   12.668%   $269.2653   $269.27   $269.25   $269.30  
  Purchase #2 $191.67   11.328%   $213.3824   $213.38   $213.40   $213.40  
  Purchase #3 $217.67   10.576%   $240.6908   $240.69   $240.70   $240.70  
  Purchase #4 $155.30   11.705%   $173.4779   $173.48   $173.50   $173.50  
  Purchase #5 $230.97   8.729%   $251.1314   $251.13   $251.15   $251.10  
  Purchase #6 $219.55   11.513%   $244.8268   $244.83   $244.85   $244.80  
  Purchase #7 $172.60   10.168%   $190.15   $190.15   $190.15   $190.20  
  7 purchase totals = $1,426.75       $1,582.9246   $1,582.93   $1,583.00   $1,583.00