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 $250.74   13.867%   $285.5101   $285.51   $285.50   $285.50  
  Purchase #2 $222.62   4.173%   $231.9099   $231.91   $231.90   $231.90  
  Purchase #3 $194.55   3.382%   $201.1297   $201.13   $201.15   $201.10  
  Purchase #4 $64.56   4.119%   $67.2192   $67.22   $67.20   $67.20  
  Purchase #5 $126.70   13.367%   $143.636   $143.64   $143.65   $143.60  
  Purchase #6 $247.67   9.291%   $270.681   $270.68   $270.70   $270.70  
  Purchase #7 $186.58   7.587%   $200.7358   $200.74   $200.75   $200.70  
  7 purchase totals = $1,293.42       $1,400.8217   $1,400.83   $1,400.85   $1,400.70