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 $249.77   12.653%   $281.3734   $281.37   $281.35   $281.40  
  Purchase #2 $136.43   4.721%   $142.8709   $142.87   $142.85   $142.90  
  Purchase #3 $5.66   3.135%   $5.8374   $5.84   $5.85   $5.80  
  Purchase #4 $125.69   3.347%   $129.8968   $129.90   $129.90   $129.90  
  Purchase #5 $180.31   6.601%   $192.2123   $192.21   $192.20   $192.20  
  Purchase #6 $191.79   12.857%   $216.4484   $216.45   $216.45   $216.40  
  Purchase #7 $31.38   9.286%   $34.2939   $34.29   $34.30   $34.30  
  7 purchase totals = $921.03       $1,002.9331   $1,002.93   $1,002.90   $1,002.90