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 $7.70   3.405%   $7.9622   $7.96   $7.95   $8.00  
  Purchase #2 $12.54   5.346%   $13.2104   $13.21   $13.20   $13.20  
  Purchase #3 $144.28   11.475%   $160.8361   $160.84   $160.85   $160.80  
  Purchase #4 $44.46   9.332%   $48.609   $48.61   $48.60   $48.60  
  Purchase #5 $186.32   13.818%   $212.0657   $212.07   $212.05   $212.10  
  Purchase #6 $115.29   4.475%   $120.4492   $120.45   $120.45   $120.40  
  Purchase #7 $29.89   13.541%   $33.9374   $33.94   $33.95   $33.90  
  7 purchase totals = $540.48       $597.0700   $597.08   $597.05   $597.00