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 $125.76   10.452%   $138.9044   $138.90   $138.90   $138.90  
  Purchase #2 $93.98   8.174%   $101.6619   $101.66   $101.65   $101.70  
  Purchase #3 $91.78   5.549%   $96.8729   $96.87   $96.85   $96.90  
  Purchase #4 $166.48   8.823%   $181.1685   $181.17   $181.15   $181.20  
  Purchase #5 $127.90   6.272%   $135.9219   $135.92   $135.90   $135.90  
  Purchase #6 $74.70   11.154%   $83.032   $83.03   $83.05   $83.00  
  Purchase #7 $214.25   7.556%   $230.4387   $230.44   $230.45   $230.40  
  7 purchase totals = $894.85       $968.0003   $967.99   $967.95   $968.00