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 $234.72   9.406%   $256.7978   $256.80   $256.80   $256.80  
  Purchase #2 $30.52   6.598%   $32.5337   $32.53   $32.55   $32.50  
  Purchase #3 $77.87   12.702%   $87.761   $87.76   $87.75   $87.80  
  Purchase #4 $23.59   5.386%   $24.8606   $24.86   $24.85   $24.90  
  Purchase #5 $229.61   3.707%   $238.1216   $238.12   $238.10   $238.10  
  Purchase #6 $187.37   11.727%   $209.3429   $209.34   $209.35   $209.30  
  Purchase #7 $90.91   4.279%   $94.8   $94.80   $94.80   $94.80  
  7 purchase totals = $874.59       $944.2176   $944.21   $944.20   $944.20