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 $116.28   3.405%   $120.2393   $120.24   $120.25   $120.20  
  Purchase #2 $18.96   5.754%   $20.051   $20.05   $20.05   $20.10  
  Purchase #3 $17.97   6.719%   $19.1774   $19.18   $19.20   $19.20  
  Purchase #4 $107.53   12.156%   $120.6013   $120.60   $120.60   $120.60  
  Purchase #5 $123.86   11.304%   $137.8611   $137.86   $137.85   $137.90  
  Purchase #6 $33.89   9.478%   $37.1021   $37.10   $37.10   $37.10  
  Purchase #7 $243.91   4.176%   $254.0957   $254.10   $254.10   $254.10  
  7 purchase totals = $662.40       $709.1279   $709.13   $709.15   $709.20