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 $235.30   5.825%   $249.0062   $249.01   $249.00   $249.00  
  Purchase #2 $182.91   6.485%   $194.7717   $194.77   $194.75   $194.80  
  Purchase #3 $35.88   5.375%   $37.8086   $37.81   $37.80   $37.80  
  Purchase #4 $191.72   12.371%   $215.4377   $215.44   $215.45   $215.40  
  Purchase #5 $132.91   11.859%   $148.6718   $148.67   $148.65   $148.70  
  Purchase #6 $173.60   12.164%   $194.7167   $194.72   $194.70   $194.70  
  Purchase #7 $177.38   13.599%   $201.5019   $201.50   $201.50   $201.50  
  7 purchase totals = $1,129.70       $1,241.9146   $1,241.92   $1,241.85   $1,241.90