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 $109.98   10.545%   $121.5774   $121.58   $121.60   $121.60  
  Purchase #2 $227.62   6.579%   $242.5951   $242.60   $242.60   $242.60  
  Purchase #3 $35.31   5.459%   $37.2376   $37.24   $37.25   $37.20  
  Purchase #4 $113.43   12.402%   $127.4976   $127.50   $127.50   $127.50  
  Purchase #5 $218.47   7.303%   $234.4249   $234.42   $234.40   $234.40  
  Purchase #6 $148.75   8.251%   $161.0234   $161.02   $161.00   $161.00  
  Purchase #7 $38.39   13.791%   $43.6844   $43.68   $43.70   $43.70  
  7 purchase totals = $891.95       $968.0404   $968.04   $968.05   $968.00