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 $8.37   7.297%   $8.9808   $8.98   $9.00   $9.00  
  Purchase #2 $22.73   5.278%   $23.9297   $23.93   $23.95   $23.90  
  Purchase #3 $178.43   12.854%   $201.3654   $201.37   $201.35   $201.40  
  Purchase #4 $37.56   6.731%   $40.0882   $40.09   $40.10   $40.10  
  Purchase #5 $166.33   13.191%   $188.2706   $188.27   $188.25   $188.30  
  Purchase #6 $130.45   4.602%   $136.4533   $136.45   $136.45   $136.50  
  Purchase #7 $103.55   11.181%   $115.1279   $115.13   $115.15   $115.10  
  7 purchase totals = $647.42       $714.2159   $714.22   $714.25   $714.30