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 $56.93   12.581%   $64.0924   $64.09   $64.10   $64.10  
  Purchase #2 $60.84   10.398%   $67.1661   $67.17   $67.15   $67.20  
  Purchase #3 $211.85   6.832%   $226.3236   $226.32   $226.30   $226.30  
  Purchase #4 $104.93   5.196%   $110.3822   $110.38   $110.40   $110.40  
  Purchase #5 $85.42   7.806%   $92.0879   $92.09   $92.10   $92.10  
  Purchase #6 $50.51   5.178%   $53.1254   $53.13   $53.15   $53.10  
  Purchase #7 $204.35   5.634%   $215.8631   $215.86   $215.85   $215.90  
  7 purchase totals = $774.83       $829.0407   $829.04   $829.05   $829.10