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 $96.68   3.797%   $100.3509   $100.35   $100.35   $100.40  
  Purchase #2 $138.28   7.848%   $149.1322   $149.13   $149.15   $149.10  
  Purchase #3 $47.68   12.717%   $53.7435   $53.74   $53.75   $53.70  
  Purchase #4 $157.77   9.874%   $173.3482   $173.35   $173.35   $173.30  
  Purchase #5 $64.33   10.588%   $71.1413   $71.14   $71.15   $71.10  
  Purchase #6 $38.44   9.136%   $41.9519   $41.95   $41.95   $42.00  
  Purchase #7 $43.59   13.616%   $49.5252   $49.53   $49.55   $49.50  
  7 purchase totals = $586.77       $639.1932   $639.19   $639.25   $639.10