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 $28.33   12.273%   $31.8069   $31.81   $31.80   $31.80  
  Purchase #2 $133.29   7.584%   $143.3987   $143.40   $143.40   $143.40  
  Purchase #3 $98.51   6.339%   $104.7545   $104.75   $104.75   $104.80  
  Purchase #4 $213.34   6.460%   $227.1218   $227.12   $227.10   $227.10  
  Purchase #5 $129.59   10.448%   $143.1296   $143.13   $143.15   $143.10  
  Purchase #6 $132.54   6.221%   $140.7853   $140.79   $140.80   $140.80  
  Purchase #7 $132.93   5.589%   $140.3595   $140.36   $140.35   $140.40  
  7 purchase totals = $868.53       $931.3563   $931.36   $931.35   $931.40