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 $244.35   13.293%   $276.8314   $276.83   $276.85   $276.80  
  Purchase #2 $73.89   8.228%   $79.9697   $79.97   $79.95   $80.00  
  Purchase #3 $111.43   5.319%   $117.357   $117.36   $117.35   $117.40  
  Purchase #4 $62.37   7.688%   $67.165   $67.17   $67.15   $67.20  
  Purchase #5 $85.81   6.381%   $91.2855   $91.29   $91.30   $91.30  
  Purchase #6 $36.81   5.160%   $38.7094   $38.71   $38.70   $38.70  
  Purchase #7 $238.67   13.335%   $270.4966   $270.50   $270.50   $270.50  
  7 purchase totals = $853.33       $941.8146   $941.83   $941.80   $941.90