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 $160.91   5.744%   $170.1527   $170.15   $170.15   $170.20  
  Purchase #2 $18.45   9.597%   $20.2206   $20.22   $20.20   $20.20  
  Purchase #3 $83.96   9.832%   $92.2149   $92.21   $92.20   $92.20  
  Purchase #4 $135.25   5.590%   $142.8105   $142.81   $142.80   $142.80  
  Purchase #5 $73.65   3.135%   $75.9589   $75.96   $75.95   $76.00  
  Purchase #6 $35.25   10.536%   $38.9639   $38.96   $38.95   $39.00  
  Purchase #7 $94.47   12.810%   $106.5716   $106.57   $106.55   $106.60  
  7 purchase totals = $601.94       $646.8931   $646.88   $646.80   $647.00