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 $26.31   7.411%   $28.2598   $28.26   $28.25   $28.30  
  Purchase #2 $5.59   6.154%   $5.934   $5.93   $5.95   $5.90  
  Purchase #3 $164.65   12.540%   $185.2971   $185.30   $185.30   $185.30  
  Purchase #4 $160.51   12.344%   $180.3234   $180.32   $180.30   $180.30  
  Purchase #5 $77.54   4.486%   $81.0184   $81.02   $81.00   $81.00  
  Purchase #6 $215.84   12.782%   $243.4287   $243.43   $243.45   $243.40  
  Purchase #7 $17.34   4.216%   $18.0711   $18.07   $18.05   $18.10  
  7 purchase totals = $667.78       $742.3325   $742.33   $742.30   $742.30