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 $74.93   5.274%   $78.8818   $78.88   $78.90   $78.90  
  Purchase #2 $203.31   11.513%   $226.7171   $226.72   $226.70   $226.70  
  Purchase #3 $75.82   12.275%   $85.1269   $85.13   $85.15   $85.10  
  Purchase #4 $93.37   9.106%   $101.8723   $101.87   $101.85   $101.90  
  Purchase #5 $144.50   4.290%   $150.6991   $150.70   $150.70   $150.70  
  Purchase #6 $188.47   3.182%   $194.4671   $194.47   $194.45   $194.50  
  Purchase #7 $135.71   11.109%   $150.786   $150.79   $150.80   $150.80  
  7 purchase totals = $916.11       $988.5503   $988.56   $988.55   $988.60