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 $150.63   13.205%   $170.5207   $170.52   $170.50   $170.50  
  Purchase #2 $38.64   3.491%   $39.9889   $39.99   $40.00   $40.00  
  Purchase #3 $97.95   12.382%   $110.0782   $110.08   $110.10   $110.10  
  Purchase #4 $187.90   5.298%   $197.8549   $197.85   $197.85   $197.90  
  Purchase #5 $99.80   4.616%   $104.4068   $104.41   $104.40   $104.40  
  Purchase #6 $84.32   10.630%   $93.2832   $93.28   $93.30   $93.30  
  Purchase #7 $149.72   8.337%   $162.2022   $162.20   $162.20   $162.20  
  7 purchase totals = $808.96       $878.3349   $878.33   $878.35   $878.40