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 $19.97   5.334%   $21.0352   $21.04   $21.05   $21.00  
  Purchase #2 $12.75   10.267%   $14.059   $14.06   $14.05   $14.10  
  Purchase #3 $65.77   11.309%   $73.2079   $73.21   $73.20   $73.20  
  Purchase #4 $12.90   10.259%   $14.2234   $14.22   $14.20   $14.20  
  Purchase #5 $219.65   6.464%   $233.8482   $233.85   $233.85   $233.80  
  Purchase #6 $5.48   5.666%   $5.7905   $5.79   $5.80   $5.80  
  Purchase #7 $138.90   13.853%   $158.1418   $158.14   $158.15   $158.10  
  7 purchase totals = $475.42       $520.3060   $520.31   $520.30   $520.20