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 $249.70   3.373%   $258.1224   $258.12   $258.10   $258.10  
  Purchase #2 $131.38   11.786%   $146.8644   $146.86   $146.85   $146.90  
  Purchase #3 $116.65   8.405%   $126.4544   $126.45   $126.45   $126.50  
  Purchase #4 $61.39   12.853%   $69.2805   $69.28   $69.30   $69.30  
  Purchase #5 $187.76   7.611%   $202.0504   $202.05   $202.05   $202.10  
  Purchase #6 $61.34   11.742%   $68.5425   $68.54   $68.55   $68.50  
  Purchase #7 $118.89   4.761%   $124.5504   $124.55   $124.55   $124.60  
  7 purchase totals = $927.11       $995.8650   $995.85   $995.85   $996.00