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 $226.35   13.107%   $256.0177   $256.02   $256.00   $256.00  
  Purchase #2 $63.89   11.220%   $71.0585   $71.06   $71.05   $71.10  
  Purchase #3 $246.35   3.493%   $254.955   $254.96   $254.95   $255.00  
  Purchase #4 $121.34   8.249%   $131.3493   $131.35   $131.35   $131.30  
  Purchase #5 $116.59   5.390%   $122.8742   $122.87   $122.85   $122.90  
  Purchase #6 $34.64   3.736%   $35.9342   $35.93   $35.95   $35.90  
  Purchase #7 $158.57   5.576%   $167.4119   $167.41   $167.40   $167.40  
  7 purchase totals = $967.73       $1,039.6008   $1,039.60   $1,039.55   $1,039.60