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 $145.92   12.755%   $164.5321   $164.53   $164.55   $164.50  
  Purchase #2 $174.81   12.694%   $197.0004   $197.00   $197.00   $197.00  
  Purchase #3 $131.43   3.117%   $135.5267   $135.53   $135.55   $135.50  
  Purchase #4 $124.57   9.177%   $136.0018   $136.00   $136.00   $136.00  
  Purchase #5 $74.42   11.535%   $83.0043   $83.00   $83.00   $83.00  
  Purchase #6 $63.96   13.367%   $72.5095   $72.51   $72.50   $72.50  
  Purchase #7 $191.47   8.381%   $207.5171   $207.52   $207.50   $207.50  
  7 purchase totals = $906.58       $996.0919   $996.09   $996.10   $996.00