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 $248.80   12.132%   $278.9844   $278.98   $279.00   $279.00  
  Purchase #2 $16.92   9.873%   $18.5905   $18.59   $18.60   $18.60  
  Purchase #3 $210.67   11.207%   $234.2798   $234.28   $234.30   $234.30  
  Purchase #4 $82.89   8.574%   $89.997   $90.00   $90.00   $90.00  
  Purchase #5 $177.91   7.651%   $191.5219   $191.52   $191.50   $191.50  
  Purchase #6 $89.40   6.216%   $94.9571   $94.96   $94.95   $95.00  
  Purchase #7 $39.46   6.248%   $41.9255   $41.93   $41.95   $41.90  
  7 purchase totals = $866.05       $950.2562   $950.26   $950.30   $950.30