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 $246.59   12.334%   $277.0044   $277.00   $277.00   $277.00  
  Purchase #2 $191.38   3.820%   $198.6907   $198.69   $198.70   $198.70  
  Purchase #3 $131.68   13.410%   $149.3383   $149.34   $149.35   $149.30  
  Purchase #4 $212.25   10.816%   $235.207   $235.21   $235.20   $235.20  
  Purchase #5 $55.75   6.328%   $59.2779   $59.28   $59.30   $59.30  
  Purchase #6 $195.41   8.703%   $212.4165   $212.42   $212.40   $212.40  
  Purchase #7 $87.80   8.194%   $94.9943   $94.99   $95.00   $95.00  
  7 purchase totals = $1,120.86       $1,226.9291   $1,226.93   $1,226.95   $1,226.90