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 $7.80   4.342%   $8.1387   $8.14   $8.15   $8.10  
  Purchase #2 $159.66   9.344%   $174.5786   $174.58   $174.60   $174.60  
  Purchase #3 $10.97   8.562%   $11.9093   $11.91   $11.90   $11.90  
  Purchase #4 $7.71   6.663%   $8.2237   $8.22   $8.20   $8.20  
  Purchase #5 $186.46   7.714%   $200.8435   $200.84   $200.85   $200.80  
  Purchase #6 $37.89   3.252%   $39.1222   $39.12   $39.10   $39.10  
  Purchase #7 $42.36   3.127%   $43.6846   $43.68   $43.70   $43.70  
  7 purchase totals = $452.85       $486.5006   $486.49   $486.50   $486.40