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 $35.45   12.755%   $39.9716   $39.97   $39.95   $40.00  
  Purchase #2 $36.49   6.395%   $38.8235   $38.82   $38.80   $38.80  
  Purchase #3 $222.46   10.236%   $245.231   $245.23   $245.25   $245.20  
  Purchase #4 $179.37   12.807%   $202.3419   $202.34   $202.35   $202.30  
  Purchase #5 $153.51   4.280%   $160.0802   $160.08   $160.10   $160.10  
  Purchase #6 $22.57   13.700%   $25.6621   $25.66   $25.65   $25.70  
  Purchase #7 $128.67   11.445%   $143.3963   $143.40   $143.40   $143.40  
  7 purchase totals = $778.52       $855.5066   $855.50   $855.50   $855.50