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 $6.28   10.106%   $6.9147   $6.91   $6.90   $6.90  
  Purchase #2 $117.32   12.560%   $132.0554   $132.06   $132.05   $132.10  
  Purchase #3 $38.45   9.586%   $42.1358   $42.14   $42.15   $42.10  
  Purchase #4 $86.81   11.853%   $97.0996   $97.10   $97.10   $97.10  
  Purchase #5 $221.36   8.504%   $240.1845   $240.18   $240.20   $240.20  
  Purchase #6 $37.88   3.718%   $39.2884   $39.29   $39.30   $39.30  
  Purchase #7 $50.82   13.869%   $57.8682   $57.87   $57.85   $57.90  
  7 purchase totals = $558.92       $615.5466   $615.55   $615.55   $615.60