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 $19.45   7.783%   $20.9638   $20.96   $20.95   $21.00  
  Purchase #2 $76.31   7.591%   $82.1027   $82.10   $82.10   $82.10  
  Purchase #3 $138.71   5.204%   $145.9285   $145.93   $145.95   $145.90  
  Purchase #4 $79.63   10.641%   $88.1034   $88.10   $88.10   $88.10  
  Purchase #5 $157.65   8.398%   $170.8894   $170.89   $170.90   $170.90  
  Purchase #6 $166.40   8.801%   $181.0449   $181.04   $181.05   $181.00  
  Purchase #7 $204.45   7.694%   $220.1804   $220.18   $220.20   $220.20  
  7 purchase totals = $842.60       $909.2131   $909.20   $909.25   $909.20