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 $244.51   12.673%   $275.4968   $275.50   $275.50   $275.50  
  Purchase #2 $17.58   13.823%   $20.0101   $20.01   $20.00   $20.00  
  Purchase #3 $224.40   4.279%   $234.0021   $234.00   $234.00   $234.00  
  Purchase #4 $23.98   6.142%   $25.4529   $25.45   $25.45   $25.50  
  Purchase #5 $193.32   13.138%   $218.7184   $218.72   $218.70   $218.70  
  Purchase #6 $162.34   3.326%   $167.7394   $167.74   $167.75   $167.70  
  Purchase #7 $91.42   5.576%   $96.5176   $96.52   $96.50   $96.50  
  7 purchase totals = $957.55       $1,037.9373   $1,037.94   $1,037.90   $1,037.90