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 $39.57   3.339%   $40.8912   $40.89   $40.90   $40.90  
  Purchase #2 $16.69   5.618%   $17.6276   $17.63   $17.65   $17.60  
  Purchase #3 $210.42   11.265%   $234.1238   $234.12   $234.10   $234.10  
  Purchase #4 $141.77   9.202%   $154.8157   $154.82   $154.80   $154.80  
  Purchase #5 $73.28   3.374%   $75.7525   $75.75   $75.75   $75.80  
  Purchase #6 $199.79   9.737%   $219.2436   $219.24   $219.25   $219.20  
  Purchase #7 $166.33   7.258%   $178.4022   $178.40   $178.40   $178.40  
  7 purchase totals = $847.85       $920.8566   $920.85   $920.85   $920.80