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 $29.76   4.112%   $30.9837   $30.98   $31.00   $31.00  
  Purchase #2 $242.44   10.440%   $267.7507   $267.75   $267.75   $267.80  
  Purchase #3 $235.83   9.278%   $257.7103   $257.71   $257.70   $257.70  
  Purchase #4 $116.79   9.204%   $127.5394   $127.54   $127.55   $127.50  
  Purchase #5 $37.88   4.333%   $39.5213   $39.52   $39.50   $39.50  
  Purchase #6 $13.58   8.846%   $14.7813   $14.78   $14.80   $14.80  
  Purchase #7 $221.85   3.850%   $230.3912   $230.39   $230.40   $230.40  
  7 purchase totals = $898.13       $968.6779   $968.67   $968.70   $968.70