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 $81.53   12.194%   $91.4718   $91.47   $91.45   $91.50  
  Purchase #2 $101.86   11.584%   $113.6595   $113.66   $113.65   $113.70  
  Purchase #3 $207.42   13.557%   $235.5399   $235.54   $235.55   $235.50  
  Purchase #4 $92.69   6.626%   $98.8316   $98.83   $98.85   $98.80  
  Purchase #5 $121.78   12.531%   $137.0403   $137.04   $137.05   $137.00  
  Purchase #6 $141.52   8.164%   $153.0737   $153.07   $153.05   $153.10  
  Purchase #7 $10.71   7.240%   $11.4854   $11.49   $11.50   $11.50  
  7 purchase totals = $757.51       $841.1022   $841.10   $841.10   $841.10