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 $183.34   11.170%   $203.8191   $203.82   $203.80   $203.80  
  Purchase #2 $53.81   12.504%   $60.5384   $60.54   $60.55   $60.50  
  Purchase #3 $118.95   6.349%   $126.5021   $126.50   $126.50   $126.50  
  Purchase #4 $94.79   11.302%   $105.5032   $105.50   $105.50   $105.50  
  Purchase #5 $171.37   3.823%   $177.9215   $177.92   $177.90   $177.90  
  Purchase #6 $121.70   12.782%   $137.2557   $137.26   $137.25   $137.30  
  Purchase #7 $180.83   4.432%   $188.8444   $188.84   $188.85   $188.80  
  7 purchase totals = $924.79       $1,000.3844   $1,000.38   $1,000.35   $1,000.30