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 $58.82   11.847%   $65.7884   $65.79   $65.80   $65.80  
  Purchase #2 $168.30   13.108%   $190.3608   $190.36   $190.35   $190.40  
  Purchase #3 $211.53   7.236%   $226.8363   $226.84   $226.85   $226.80  
  Purchase #4 $23.51   12.858%   $26.5329   $26.53   $26.55   $26.50  
  Purchase #5 $148.69   11.839%   $166.2934   $166.29   $166.30   $166.30  
  Purchase #6 $106.64   12.606%   $120.083   $120.08   $120.10   $120.10  
  Purchase #7 $171.75   12.661%   $193.4953   $193.50   $193.50   $193.50  
  7 purchase totals = $889.24       $989.3901   $989.39   $989.45   $989.40