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 $37.49   12.746%   $42.2685   $42.27   $42.25   $42.30  
  Purchase #2 $20.25   8.821%   $22.0363   $22.04   $22.05   $22.00  
  Purchase #3 $49.81   7.566%   $53.5786   $53.58   $53.60   $53.60  
  Purchase #4 $9.35   9.680%   $10.2551   $10.26   $10.25   $10.30  
  Purchase #5 $200.91   5.526%   $212.0123   $212.01   $212.00   $212.00  
  Purchase #6 $57.84   5.609%   $61.0842   $61.08   $61.10   $61.10  
  Purchase #7 $45.45   4.762%   $47.6143   $47.61   $47.60   $47.60  
  7 purchase totals = $421.10       $448.8493   $448.85   $448.85   $448.90