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 $27.76   5.550%   $29.3007   $29.30   $29.30   $29.30  
  Purchase #2 $53.97   5.141%   $56.7446   $56.74   $56.75   $56.70  
  Purchase #3 $115.58   13.361%   $131.0226   $131.02   $131.00   $131.00  
  Purchase #4 $235.33   8.172%   $254.5612   $254.56   $254.55   $254.60  
  Purchase #5 $221.85   11.120%   $246.5197   $246.52   $246.50   $246.50  
  Purchase #6 $236.96   12.323%   $266.1606   $266.16   $266.15   $266.20  
  Purchase #7 $92.35   11.566%   $103.0312   $103.03   $103.05   $103.00  
  7 purchase totals = $983.80       $1,087.3406   $1,087.33   $1,087.30   $1,087.30