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 $133.37   4.351%   $139.1729   $139.17   $139.15   $139.20  
  Purchase #2 $19.48   4.362%   $20.3297   $20.33   $20.35   $20.30  
  Purchase #3 $7.76   10.798%   $8.5979   $8.60   $8.60   $8.60  
  Purchase #4 $25.42   6.114%   $26.9742   $26.97   $26.95   $27.00  
  Purchase #5 $110.40   10.105%   $121.5559   $121.56   $121.55   $121.60  
  Purchase #6 $181.64   6.454%   $193.363   $193.36   $193.35   $193.40  
  Purchase #7 $23.68   4.813%   $24.8197   $24.82   $24.80   $24.80  
  7 purchase totals = $501.75       $534.8133   $534.81   $534.75   $534.90