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 $250.74   7.846%   $270.4131   $270.41   $270.40   $270.40  
  Purchase #2 $96.73   5.559%   $102.1072   $102.11   $102.10   $102.10  
  Purchase #3 $192.68   7.866%   $207.8362   $207.84   $207.85   $207.80  
  Purchase #4 $219.86   8.487%   $238.5195   $238.52   $238.50   $238.50  
  Purchase #5 $18.41   12.534%   $20.7175   $20.72   $20.70   $20.70  
  Purchase #6 $52.48   10.336%   $57.9043   $57.90   $57.90   $57.90  
  Purchase #7 $137.51   11.312%   $153.0651   $153.07   $153.05   $153.10  
  7 purchase totals = $968.41       $1,050.5629   $1,050.57   $1,050.50   $1,050.50