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 $244.97   11.171%   $272.3356   $272.34   $272.35   $272.30  
  Purchase #2 $179.80   8.108%   $194.3782   $194.38   $194.40   $194.40  
  Purchase #3 $129.34   11.418%   $144.108   $144.11   $144.10   $144.10  
  Purchase #4 $58.89   5.110%   $61.8993   $61.90   $61.90   $61.90  
  Purchase #5 $202.76   11.400%   $225.8746   $225.87   $225.85   $225.90  
  Purchase #6 $123.29   9.406%   $134.8867   $134.89   $134.90   $134.90  
  Purchase #7 $212.86   10.166%   $234.4993   $234.50   $234.50   $234.50  
  7 purchase totals = $1,151.91       $1,267.9817   $1,267.99   $1,268.00   $1,268.00