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 $80.40   8.817%   $87.4889   $87.49   $87.50   $87.50  
  Purchase #2 $17.33   12.749%   $19.5394   $19.54   $19.55   $19.50  
  Purchase #3 $125.53   10.532%   $138.7508   $138.75   $138.75   $138.80  
  Purchase #4 $9.51   7.338%   $10.2078   $10.21   $10.20   $10.20  
  Purchase #5 $243.45   8.496%   $264.1335   $264.13   $264.15   $264.10  
  Purchase #6 $25.44   3.599%   $26.3556   $26.36   $26.35   $26.40  
  Purchase #7 $83.96   11.857%   $93.9151   $93.92   $93.90   $93.90  
  7 purchase totals = $585.62       $640.3911   $640.40   $640.40   $640.40