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 $19.79   6.484%   $21.0732   $21.07   $21.05   $21.10  
  Purchase #2 $147.85   7.628%   $159.128   $159.13   $159.15   $159.10  
  Purchase #3 $196.61   13.183%   $222.5291   $222.53   $222.55   $222.50  
  Purchase #4 $192.48   12.793%   $217.104   $217.10   $217.10   $217.10  
  Purchase #5 $211.96   4.477%   $221.4494   $221.45   $221.45   $221.40  
  Purchase #6 $137.90   8.506%   $149.6298   $149.63   $149.65   $149.60  
  Purchase #7 $76.68   4.374%   $80.034   $80.03   $80.05   $80.00  
  7 purchase totals = $983.27       $1,070.9475   $1,070.94   $1,071.00   $1,070.80