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 $6.99   4.585%   $7.3105   $7.31   $7.30   $7.30  
  Purchase #2 $145.31   3.781%   $150.8042   $150.80   $150.80   $150.80  
  Purchase #3 $162.74   13.856%   $185.2893   $185.29   $185.30   $185.30  
  Purchase #4 $56.92   5.459%   $60.0273   $60.03   $60.05   $60.00  
  Purchase #5 $6.44   9.415%   $7.0463   $7.05   $7.05   $7.00  
  Purchase #6 $19.69   3.182%   $20.3165   $20.32   $20.30   $20.30  
  Purchase #7 $226.88   12.391%   $254.9927   $254.99   $255.00   $255.00  
  7 purchase totals = $624.97       $685.7868   $685.79   $685.80   $685.70