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 $234.56   11.281%   $261.0207   $261.02   $261.00   $261.00  
  Purchase #2 $81.69   7.524%   $87.8364   $87.84   $87.85   $87.80  
  Purchase #3 $5.26   10.489%   $5.8117   $5.81   $5.80   $5.80  
  Purchase #4 $96.65   9.156%   $105.4993   $105.50   $105.50   $105.50  
  Purchase #5 $176.75   3.677%   $183.2491   $183.25   $183.25   $183.20  
  Purchase #6 $243.29   3.349%   $251.4378   $251.44   $251.45   $251.40  
  Purchase #7 $49.98   6.165%   $53.0613   $53.06   $53.05   $53.10  
  7 purchase totals = $888.18       $947.9163   $947.92   $947.90   $947.80