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 $36.98   13.556%   $41.993   $41.99   $42.00   $42.00  
  Purchase #2 $62.35   4.278%   $65.0173   $65.02   $65.00   $65.00  
  Purchase #3 $51.39   6.498%   $54.7293   $54.73   $54.75   $54.70  
  Purchase #4 $70.69   7.673%   $76.114   $76.11   $76.10   $76.10  
  Purchase #5 $67.62   6.457%   $71.9862   $71.99   $72.00   $72.00  
  Purchase #6 $240.44   4.726%   $251.8032   $251.80   $251.80   $251.80  
  Purchase #7 $39.30   7.569%   $42.2746   $42.27   $42.25   $42.30  
  7 purchase totals = $568.77       $603.9176   $603.91   $603.90   $603.90