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 $79.64   11.458%   $88.7652   $88.77   $88.75   $88.80  
  Purchase #2 $89.46   11.722%   $99.9465   $99.95   $99.95   $99.90  
  Purchase #3 $189.90   11.860%   $212.4221   $212.42   $212.40   $212.40  
  Purchase #4 $7.25   11.123%   $8.0564   $8.06   $8.05   $8.10  
  Purchase #5 $23.96   13.120%   $27.1036   $27.10   $27.10   $27.10  
  Purchase #6 $41.47   6.233%   $44.0548   $44.05   $44.05   $44.10  
  Purchase #7 $55.80   4.605%   $58.3696   $58.37   $58.35   $58.40  
  7 purchase totals = $487.48       $538.7182   $538.72   $538.65   $538.80