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 $27.25   7.730%   $29.3564   $29.36   $29.35   $29.40  
  Purchase #2 $21.33   5.792%   $22.5654   $22.57   $22.55   $22.60  
  Purchase #3 $227.39   8.672%   $247.1093   $247.11   $247.10   $247.10  
  Purchase #4 $102.53   10.183%   $112.9706   $112.97   $112.95   $113.00  
  Purchase #5 $168.93   13.216%   $191.2558   $191.26   $191.25   $191.30  
  Purchase #6 $5.82   7.553%   $6.2596   $6.26   $6.25   $6.30  
  Purchase #7 $37.57   7.109%   $40.2409   $40.24   $40.25   $40.20  
  7 purchase totals = $590.82       $649.7580   $649.77   $649.70   $649.90