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 $36.71   5.736%   $38.8157   $38.82   $38.80   $38.80  
  Purchase #2 $19.66   7.860%   $21.2053   $21.21   $21.20   $21.20  
  Purchase #3 $198.64   4.447%   $207.4735   $207.47   $207.45   $207.50  
  Purchase #4 $149.95   3.759%   $155.5866   $155.59   $155.60   $155.60  
  Purchase #5 $17.99   4.266%   $18.7575   $18.76   $18.75   $18.80  
  Purchase #6 $64.61   6.399%   $68.7444   $68.74   $68.75   $68.70  
  Purchase #7 $145.60   10.425%   $160.7788   $160.78   $160.80   $160.80  
  7 purchase totals = $633.16       $671.3618   $671.37   $671.35   $671.40