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 $78.44   6.440%   $83.4915   $83.49   $83.50   $83.50  
  Purchase #2 $6.79   4.463%   $7.093   $7.09   $7.10   $7.10  
  Purchase #3 $232.73   11.107%   $258.5793   $258.58   $258.60   $258.60  
  Purchase #4 $128.84   12.762%   $145.2826   $145.28   $145.30   $145.30  
  Purchase #5 $75.59   8.493%   $82.0099   $82.01   $82.00   $82.00  
  Purchase #6 $62.52   10.486%   $69.0758   $69.08   $69.10   $69.10  
  Purchase #7 $30.39   13.872%   $34.6057   $34.61   $34.60   $34.60  
  7 purchase totals = $615.30       $680.1378   $680.14   $680.20   $680.20