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 $248.96   10.789%   $275.8203   $275.82   $275.80   $275.80  
  Purchase #2 $18.40   9.390%   $20.1278   $20.13   $20.15   $20.10  
  Purchase #3 $229.55   7.536%   $246.8489   $246.85   $246.85   $246.80  
  Purchase #4 $108.93   9.182%   $118.932   $118.93   $118.95   $118.90  
  Purchase #5 $145.27   10.347%   $160.3011   $160.30   $160.30   $160.30  
  Purchase #6 $210.85   6.467%   $224.4857   $224.49   $224.50   $224.50  
  Purchase #7 $197.86   13.226%   $224.029   $224.03   $224.05   $224.00  
  7 purchase totals = $1,159.82       $1,270.5448   $1,270.55   $1,270.60   $1,270.40