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 $124.43   11.332%   $138.5304   $138.53   $138.55   $138.50  
  Purchase #2 $87.95   4.308%   $91.7389   $91.74   $91.75   $91.70  
  Purchase #3 $31.61   9.173%   $34.5096   $34.51   $34.50   $34.50  
  Purchase #4 $135.40   7.457%   $145.4968   $145.50   $145.50   $145.50  
  Purchase #5 $63.30   4.143%   $65.9225   $65.92   $65.90   $65.90  
  Purchase #6 $95.69   6.580%   $101.9864   $101.99   $102.00   $102.00  
  Purchase #7 $210.25   3.397%   $217.3922   $217.39   $217.40   $217.40  
  7 purchase totals = $748.63       $795.5768   $795.58   $795.60   $795.50