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 $73.87   13.818%   $84.0774   $84.08   $84.10   $84.10  
  Purchase #2 $179.37   12.801%   $202.3312   $202.33   $202.35   $202.30  
  Purchase #3 $229.84   11.576%   $256.4463   $256.45   $256.45   $256.40  
  Purchase #4 $151.73   11.588%   $169.3125   $169.31   $169.30   $169.30  
  Purchase #5 $85.46   7.155%   $91.5747   $91.57   $91.55   $91.60  
  Purchase #6 $180.48   13.472%   $204.7943   $204.79   $204.80   $204.80  
  Purchase #7 $222.74   12.159%   $249.823   $249.82   $249.80   $249.80  
  7 purchase totals = $1,123.49       $1,258.3594   $1,258.35   $1,258.35   $1,258.30