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 $19.50   5.139%   $20.5021   $20.50   $20.50   $20.50  
  Purchase #2 $19.44   10.110%   $21.4054   $21.41   $21.40   $21.40  
  Purchase #3 $62.78   9.207%   $68.5602   $68.56   $68.55   $68.60  
  Purchase #4 $85.81   6.604%   $91.4769   $91.48   $91.50   $91.50  
  Purchase #5 $85.50   11.180%   $95.0589   $95.06   $95.05   $95.10  
  Purchase #6 $42.40   3.424%   $43.8518   $43.85   $43.85   $43.90  
  Purchase #7 $156.33   9.149%   $170.6326   $170.63   $170.65   $170.60  
  7 purchase totals = $471.76       $511.4879   $511.49   $511.50   $511.60