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 $147.30   13.227%   $166.7834   $166.78   $166.80   $166.80  
  Purchase #2 $15.66   11.130%   $17.403   $17.40   $17.40   $17.40  
  Purchase #3 $126.92   6.795%   $135.5442   $135.54   $135.55   $135.50  
  Purchase #4 $67.39   5.339%   $70.988   $70.99   $71.00   $71.00  
  Purchase #5 $246.92   12.703%   $278.2862   $278.29   $278.30   $278.30  
  Purchase #6 $235.99   8.729%   $256.5896   $256.59   $256.60   $256.60  
  Purchase #7 $120.67   13.674%   $137.1704   $137.17   $137.15   $137.20  
  7 purchase totals = $960.85       $1,062.7648   $1,062.76   $1,062.80   $1,062.80