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 $250.93   13.150%   $283.9273   $283.93   $283.95   $283.90  
  Purchase #2 $126.55   6.266%   $134.4796   $134.48   $134.50   $134.50  
  Purchase #3 $85.78   6.777%   $91.5933   $91.59   $91.60   $91.60  
  Purchase #4 $197.89   5.285%   $208.3485   $208.35   $208.35   $208.30  
  Purchase #5 $128.82   9.202%   $140.674   $140.67   $140.65   $140.70  
  Purchase #6 $126.94   5.407%   $133.8036   $133.80   $133.80   $133.80  
  Purchase #7 $7.74   12.281%   $8.6905   $8.69   $8.70   $8.70  
  7 purchase totals = $924.65       $1,001.5168   $1,001.51   $1,001.55   $1,001.50