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 $129.40   4.578%   $135.3239   $135.32   $135.30   $135.30  
  Purchase #2 $109.80   6.757%   $117.2192   $117.22   $117.20   $117.20  
  Purchase #3 $240.57   7.277%   $258.0763   $258.08   $258.10   $258.10  
  Purchase #4 $10.30   10.847%   $11.4172   $11.42   $11.40   $11.40  
  Purchase #5 $143.90   10.660%   $159.2397   $159.24   $159.25   $159.20  
  Purchase #6 $33.32   8.128%   $36.0282   $36.03   $36.05   $36.00  
  Purchase #7 $145.42   7.342%   $156.0967   $156.10   $156.10   $156.10  
  7 purchase totals = $812.71       $873.4012   $873.41   $873.40   $873.30