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 $27.68   6.132%   $29.3773   $29.38   $29.40   $29.40  
  Purchase #2 $5.40   10.497%   $5.9668   $5.97   $5.95   $6.00  
  Purchase #3 $136.45   6.553%   $145.3916   $145.39   $145.40   $145.40  
  Purchase #4 $7.68   7.481%   $8.2545   $8.25   $8.25   $8.30  
  Purchase #5 $135.44   12.806%   $152.7844   $152.78   $152.80   $152.80  
  Purchase #6 $6.82   7.189%   $7.3103   $7.31   $7.30   $7.30  
  Purchase #7 $117.36   7.806%   $126.5211   $126.52   $126.50   $126.50  
  7 purchase totals = $436.83       $475.6060   $475.60   $475.60   $475.70