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 $70.61   3.682%   $73.2099   $73.21   $73.20   $73.20  
  Purchase #2 $218.37   6.534%   $232.6383   $232.64   $232.65   $232.60  
  Purchase #3 $62.30   12.771%   $70.2563   $70.26   $70.25   $70.30  
  Purchase #4 $184.72   9.631%   $202.5104   $202.51   $202.50   $202.50  
  Purchase #5 $129.47   6.872%   $138.3672   $138.37   $138.35   $138.40  
  Purchase #6 $23.83   4.744%   $24.9605   $24.96   $24.95   $25.00  
  Purchase #7 $238.74   3.104%   $246.1505   $246.15   $246.15   $246.20  
  7 purchase totals = $928.04       $988.0931   $988.10   $988.05   $988.20