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 $15.69   6.317%   $16.6811   $16.68   $16.70   $16.70  
  Purchase #2 $209.39   9.618%   $229.5291   $229.53   $229.55   $229.50  
  Purchase #3 $206.33   11.543%   $230.1467   $230.15   $230.15   $230.10  
  Purchase #4 $69.29   6.161%   $73.559   $73.56   $73.55   $73.60  
  Purchase #5 $157.74   11.516%   $175.9053   $175.91   $175.90   $175.90  
  Purchase #6 $197.67   11.810%   $221.0148   $221.01   $221.00   $221.00  
  Purchase #7 $67.80   6.742%   $72.3711   $72.37   $72.35   $72.40  
  7 purchase totals = $923.91       $1,019.2071   $1,019.21   $1,019.20   $1,019.20