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 $28.89   9.240%   $31.5594   $31.56   $31.55   $31.60  
  Purchase #2 $116.64   10.679%   $129.096   $129.10   $129.10   $129.10  
  Purchase #3 $143.94   3.367%   $148.7865   $148.79   $148.80   $148.80  
  Purchase #4 $23.51   9.410%   $25.7223   $25.72   $25.70   $25.70  
  Purchase #5 $208.73   8.130%   $225.6997   $225.70   $225.70   $225.70  
  Purchase #6 $172.81   11.266%   $192.2788   $192.28   $192.30   $192.30  
  Purchase #7 $82.93   4.636%   $86.7746   $86.77   $86.75   $86.80  
  7 purchase totals = $777.45       $839.9173   $839.92   $839.90   $840.00